Optical disc device

ABSTRACT

An optical disc device of the present invention includes spherical aberration changing means for changing spherical aberration occurring on a converging position of a light beam converged by a lens, an actuator for moving the spherical aberration changing means in a relatively precise manner, and an actuator for moving the spherical aberration changing means in a relatively rough manner. The optical disc device drives a second actuator and a third actuator based on a signal of spherical aberration detecting means and performs control so that spherical aberration is almost 0. The third actuator moves the spherical aberration changing means based on a direct current component included in a signal of the spherical aberration detecting means, and the second actuator moves the spherical aberration changing means based on an alternating current component included in the signal of the spherical aberration detecting means.

TECHNICAL FIELD

[0001] The present invention relates to an optical disc device orapparatus which records and reproduces optical information. The presentinvention particularly relates to an optical disc device which canaccurately correct spherical aberration occurring on a convergingposition of a light beam and perform data recording and reproductionwith a high density even if a lens having a large numerical aperture isused to converge the light beam.

BACKGROUND ART

[0002] As storage mediums for storing video information, voiceinformation, or data including computer programs, various opticalstorage mediums have been conventionally proposed which include aso-called read-only optical disc, a phase-change optical disc, amagneto-optical disc, or an optical card.

[0003] Optical disc devices are used to write data on such opticalstorage mediums (hereinafter, referred to as “optical discs”) or readdata recorded on optical discs. In the present specification, an opticaldisc device widely includes not only an optical disc drive but alsovarious kinds of apparatuses capable of writing data on an optical discand reading data from an optical disc. Namely, an “optical disc device”of the present specification includes, for example, a game machine,audio-visuals, a personal computer, and so on. Additionally, the opticaldisc device also includes a personal digital assistant (PDA) in whichdata can be written/read on/from a small optical disc.

[0004] Referring to FIG. 1, the configuration of the optical disc willbe firstly discussed below. An optical disc 20 of FIG. 1 comprises, fromthe side irradiated with a light beam by the optical head, a substrate21 made of a transparent material permitting the passage of a lightbeam, an information layer 29 for recording and reproducing data, and aprotective layer 25 for protecting the disc. The substrate 21 also has afunction of protecting data from a flaw or crack, contamination, and soon of a disk just like the protective layer 25. Besides, the “substrate”and the “protective layer” both indicate transparent members existingbetween the information layer of the optical disc and the atmosphere inthe present specification. Therefore it is not necessary to distinguishbetween the “substrate” and the “protective layer” according to amaterial, a thickness, a manufacturing method thereof. Therefore, anoptical head may be disposed on the side of the protective layer and amember represented as a “substrate” and a member represented as a“protective layer” may be replaced with each other in the presentspecification.

[0005]FIG. 2 is a perspective view schematically showing an enlargedinformation layer 29 of the optical disc 20. A light beam is emitted tothe disc 20 from the upper side of FIG. 20. As shown in FIG. 2, convextracks 28 are formed on the information layer 29 of the optical disc 20.The tracks 28 are formed concentrically or spirally with respect to thecenter of the disc. The tracks 28 may be wobbled. Information such asaddress information can be previously recorded on the optical disc 20according to the wobbling shape and the wobbling frequency of the tracks28.

[0006]FIG. 3 is a block diagram showing the configuration of aconventional optical disc device. The optical disc 20 is rotated by adisc motor 10 with a predetermined number of revolutions. A light beamemitted from a light source 3 such as a semiconductor laser, which actsas light beam irradiating means, is converged onto the information layer29 of the optical disc 20 by an objective lens 1, which acts asconverging means, and the light beam forms a light beam spot on adesired converging position on the information layer 29.

[0007] An optical system including the objective lens 1 is designed sothat fixed spherical aberration correction is performed on theassumption that focus control is stably performed on the informationlayer 29 of the optical disc 20. Namely, optical design for minimizingspherical aberration is made according to the thickness of the substrate21 of the optical disc 20. This is because dynamic correction is notnecessary for spherical aberration in the conventional optical discdevice.

[0008] Light reflected from the optical disc 20 is received by alight-receiving part 4 and photocurrent is generated according to aquantity of the received light.

[0009] The optical disc device comprises a focus actuator 2 and atracking actuator 27. The focus actuator 2 moves the objective lens 1substantially perpendicularly to the information layer 29 of the opticaldisc 20 to change the converging position of a light beam. The trackingactuator 27 moves the objective lens 1 in the radius direction of theoptical disc 20 to permit the converging position of the light beam tocorrectly follow the tracks 28 on the information layer 29 of theoptical disc 20.

[0010] The objective lens 1, the focus actuator 2, the light source 3,and the light-receiving part 4 are integrated into a module serving asan optical head 5. The optical head 5 can be moved in the radiusdirection of the optical disc 20 by a transfer table 60 acting assearching means. The transfer table 60 is driven by an output signal(driving signal) from a transfer table driving circuit 62.

[0011] Subsequently, focus control in the optical disc device will bediscussed below.

[0012] A light beam generated by the light source 3 such as asemiconductor laser is converged on the information layer 29 of theoptical disc 20 by the objective lens 1 and the light beam forms a lightbeam spot. Reflected light of the light beam spot from the optical disc20 is inputted again to the light-receiving part 4 via the object lens1.

[0013] The light-receiving part 4 is divided into four areas.Photocurrent is generated according to a light quantity detected in eachof the areas and the photocurrent is outputted to a preamplifier 11. Thepreamplifier 11 comprises I/V converters. Photocurrent inputted from thelight-receiving part 4 to the preamplifier 11 is converted into voltageby the I/V converters. Each converted signal is transmitted to a focuserror signal generator 7 and a tracking error signal generator 18. Thefocus error signal generator 7 generates, from an output signal of thepreamplifier 11, an error signal of the optical disc 20 and a light beamspot, which is outputted from the optical disc 5 and is focused, withrespect to the vertical direction.

[0014] The optical system generally comprises a focus error detectingsystem using the astigmatic method and a tracking error detecting systemusing the push-pull method.

[0015] The focus error signal generator 7 generates a focus error signal(hereinafter, referred to as an FE signal) based on an input signalaccording to the astigmatic method. The FE signal, which is an outputsignal of the focus error signal generator 7, is subjected to afiltering operation such as phase compensation and gain compensation inthe focus control section 17 and then the FE signal is outputted to afocus actuator driving circuit 9.

[0016] The objective lens 1 is driven by the focus actuator 2 based on adriving signal from the focus actuator driving circuit 9. As a result,the light beam spot is driven so as to have a predetermined convergingstate on the information layer 29 of the optical disc 20 and thus focuscontrol is achieved.

[0017] The following will discuss tracking control in the optical discdevice.

[0018] From an output signal of the preamplifier 11, the tracking errorsignal generator 18 generates, with respect to the radius direction ofthe optical disc 20, an error signal between the tracks 28 and a lightbeam spot which is outputted and focused from the optical head 5. Thetracking error signal generator 18 generates a tracking error signal(hereinafter, referred to as a TE signal) based on an input signalaccording to the push-pull method. The TE signal, which is an outputsignal of the tracking error signal generator 18, is subjected to afiltering operation such as phase compensation and gain compensation ina tracking control section 19 and then the TE signal is outputted to atracking actuator driving circuit 26.

[0019] The objective lens 1 is driven by a tracking actuator 27 based ona driving signal outputted from the tracking actuator driving circuit26. As a result, the light beam spot is driven so as to follow thetracks 28 on the information layer 29 of the optical disc 20 and thustracking control is achieved.

[0020] Referring to FIG. 4, the following will specifically describe thegeneration of the focus error signal and the tracking error signal.

[0021] As shown in FIG. 4, the light-receiving part 4 is divided intofour areas A, B, C, and D. The areas A to D of the light-receiving part4 generate photocurrent according to a light quantity detected in eachof the areas and outputs the photocurrent to corresponding I/V converter6 a, I/V converter 6 b, I/V converter 6 c, and I/V converter 6 d, whichare included in the preamplifier 11.

[0022] Signals having been converted from current to voltage by the I/Vconverter 6 a, the I/V converter 6 b, the I/V converter 6 c, and the I/Vconverter 6 d are transmitted to the focus error signal generator 7 andthe tracking error signal generator 18.

[0023] The “information track longitudinal direction” shown in FIG. 4 isa direction tangential to the tracks 28 of the optical disc 20, and the“optical disc radius direction” is a direction perpendicular to thetracks 28 of the optical disc 20. Therefore, in the focus error signalgenerator 7, the sum of the output of the I/V converter 6 b and theoutput of the I/V converter 6 d is subtracted from the sum of the outputof the I/V converter 6 a and the output of the I/V converter 6 c, sothat an FE signal is acquired by the astigmatic method.

[0024] In the tracking error signal generator 18, the sum of the outputof the I/V converter 6 b and the output of the I/V converter 6 c issubtracted from the sum of the output of the I/V converter 6 a and theoutput of the I/V converter 6 d, so that a TE signal is acquired by thepush-pull method.

[0025] In this way, the conventional optical disc device performs focuscontrol and tracking control when information is written on the opticaldisc and/or information is read from the optical disc.

[0026] However, in the conventional optical disc device, it has becomedifficult to write/read information by using a high-density opticaldisc. This point will be discussed in detail.

[0027] In recent years an objective lens with a numerical aperture (NA)larger than 0.6 and a light source with a wavelength shorter than 650 nmhave been proposed to further increase a recording density and acapacity of an optical disc. For example, a disc is proposed which has anumerical aperture of 0.85, a light source with a wavelength of 405 nm,a substrate (or a protective layer) with a thickness of 0.1 mm, and acapacity of 20 to 25 GB. Since a laser beam diameter (spot diameter) onthe optical disc is proportionate to λ/NA, it is preferable to reduce λand increase NA in view of improvement of a recording density, where λrepresents a wavelength of a laser beam.

[0028] When NA is 0.85 and the light source has a wavelength of 405 nm,although a beam spot is reduced, the aberration of a light beam,particularly spherical aberration becomes too large to neglect. Thespherical aberration is caused by the object lens and the substrate (orthe protective layer) constituting the optical disc.

[0029] As shown in FIG. 1, the information layer 29 of the optical disc20 is protected by the substrate 21. A light beam outputted from theoptical head 5 passes through the substrate 21 and forms a light beamspot on the information layer 29.

[0030] In conventional DVDs using optical systems with an NA of 0.6, achange in spherical aberration caused by an uneven thickness of thesubstrate 21 is within a tolerance and thus the change is negligible.However, when the substrate 21 has an even thickness, the light beamspot has spherical aberration proportionate to the fourth power of theNA. Thus, when the NA is increased to 0.85, a change in sphericalaberration becomes too large to neglect.

[0031] In a DVD standard, a double-layer disc having two informationrecording surfaces is also adopted to increase a recording capacity foreach optical disc. FIG. 5 is a diagram showing an example of theconfiguration of the double-layer disc. As shown in FIG. 5, thedouble-layer disc comprises, from the side of an optical head, asubstrate 21, an L0 layer (first information recording surface) 22, aspacer layer 24, an L1 layer (second information recording surface) 23,and a protective layer 25 on the back. The substrate 21 and the spacerlayer 24 are composed of a transparent medium such as a resin.

[0032] According to the multi-layer structure of FIG. 5, on the opticaldisc 20 having more than one information recording surface, it isnecessary to move the focal position of a light beam from theinformation recording surface, on which a light beam spot is currentlypositioned, to an adjacent information recording surface. Such amovement of the focal position of a light beam between the differentinformation recording surfaces will be referred to as “interlayermovement” in the following description. Referring to FIGS. 3 and 6, themethod of interlayer movement will be discussed below.

[0033] First, the following will describe the case where the focus of alight beam is moved from the information recording surface close to theobjective lens 1 of the optical head 5 to the information recordingsurface far from the objective lens 1. A microcomputer 8 stops focuscontrol once and simultaneously outputs, to the focus actuator drivingcircuit 9, an acceleration pulse for moving the objective lens 1. Theacceleration pulse has a waveform of FIG. 6(a) and moves the objectivelens 1 to the back (that is, to the information recording surface farfrom the objective lens 1).

[0034] Then, the microcomputer 8 compares a deceleration start level andan FE signal of the focus error signal generator 7. When the FE signalexceeds the deceleration start level, the microcomputer 8 outputs adeceleration pulse. When the output of the deceleration pulse iscompleted in the end, focus control is resumed.

[0035] The following will describe the case where the focus of a lightbeam is moved from the information recording surface far from theobjective lens 1 of the optical head 5 to the information recordingsurface close to the objective lens 1. In this case, the accelerationpulse/deceleration pulse with the waveforms of FIG. 6(b) is applied bythe same method, so that the focus of a light beam can be moved betweenlayers.

[0036] A higher recording density and a larger capacity are alsodemanded regarding the double-layer disc. In order to meet such ademand, the numerical aperture of the objective lens needs to exceed 0.6and the light source needs to have a wavelength shorter than 650 nm.

[0037] In the case of the double-layer disc, since the spacer layer 24is provided between the L0 layer 22 and the L1 layer 23, regarding athickness from the surface of the optical disc 20 on the side of theoptical head to the information recording surface, the L1 layer 23 islarger in thickness than the L0 layer 22 by the thickness of the spacerlayer 24. Such a difference in thickness causes spherical aberration. Inan optical system of a DVD standard where the NA of the objective lensis 0.6, the spherical aberration is within a tolerance and thusinformation can be recorded and reproduced without correctingaberration. As described above, in the case where an objective lenshaving a larger NA (e.g., 0.8 or more) is used, when the objective lensis adjusted on one of the information recording surfaces, sphericalaberration caused by the thickness of the spacer layer 24 on the otherinformation recording surface cannot be negligible.

[0038] Namely, when the NA of the objective lens exceeds 0.6, theconventional optical disc device cannot record information or reproducerecorded information on both of the information recording surfaces.

[0039] When the NA exceeds 0.6 (e.g., to 0.8 or larger), the provisionof a spherical aberration correction lens unit 15 in FIG. 7 can beconsidered. The spherical aberration correction lens unit 15 istypically composed of a pair of lenses. A relative distance between thepair of lenses is changed by moving one of the lenses. By using such aspherical aberration correction lens unit 15, whenrecording/reproduction are performed on the double-layer disc, it ispossible to correct spherical aberration in a proper manner for theinformation recording surfaces, thereby eliminating spherical aberrationcaused by the spacer layer.

[0040] The spherical aberration correction lens unit 15 is driven by aplate spring. In this case, while quick response is achieved and controlis performed with high accuracy, the spherical aberration correctionlens unit 15 moves just in a narrow range and results in a narrowcorrectable range for spherical aberration. Particularly when an uneventhickness of the substrate, the uneven characteristics of the objectivelens, and the uneven characteristics of the spherical aberrationcorrection lens unit 15 are considered, the double-layer disc lacks acorrection range, so that recording and reproduction cannot be performedin a proper manner.

[0041] In view of the above problems, an object of the present inventionis to provide an optical disc device which is capable of stablyrecording or reproducing information even when an optical disc includesa substrate (or a protective layer) having an uneven thickness causingspherical aberration.

[0042] Another object of the present invention is to provide an opticaldisc device which performs spherical aberration control with quickresponse and a wide correction range for spherical aberration, even whenthe NA of the objective lens is increased more than the conventional NA(e.g., 0.8 or larger), so that recording/reproduction can be performedon a high-density and large-capacity optical disc.

DISCLOSURE OF I/VVENTION

[0043] According to one aspect of the invention, an optical disc deviceis provided which comprises: light beam emitting means for emitting alight beam, converging means for converging the light beam toward aninformation storage medium, a first actuator for moving the convergingmeans substantially perpendicularly to an information layer of theinformation storage medium to change a converging position of the lightbeam, spherical aberration changing means for changing sphericalaberration occurring on a converging position of the light beamconverged by the converging means, a second actuator for moving thespherical aberration changing means in a relatively precise manner, athird actuator for moving the spherical aberration changing means in arelatively rough manner, light-receiving means for receiving lightreflected from the information storage medium of the light beam,converging state detecting means for detecting a signal according to aconverging state on the information layer of the information storagemedium of the light beam based on a signal of the light-receiving means,focus control means for driving the first actuator based on a signal ofthe converging state detecting means and performing control so that thelight beam is converged on a desired position of the information layerof the information storage medium, spherical aberration detecting meansfor detecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, and spherical aberration control means fordriving the second actuator and the third actuator based on a signal ofthe spherical aberration detecting means and performing control so thatspherical aberration is almost 0, wherein the third actuator moves thespherical aberration changing means at least based on a direct currentcomponent included in the signal of the spherical aberration detectingmeans, and the second actuator moves the spherical aberration changingmeans based on an alternating current component included in the signalof the spherical aberration detecting means.

[0044] In a preferred embodiment, the spherical aberration control meansdivides a control band so that the third actuator is driven when achange in spherical aberration is equal to or lower than a rotationalfrequency of the information storage medium, and the second actuator isdriven when a change in spherical aberration is equal to or higher thanthe rotational frequency of the information storage medium.

[0045] According to another aspect of the invention, an optical discdevice for recording data on an information storage medium having atleast two laminated information layers and/or reproducing data from theinformation storage medium is provided, which comprises: light beamemitting means for emitting a light beam, converging means forconverging the light beam toward the information storage medium, a firstactuator for moving the converging means substantially perpendicularlyto the information layer of the information storage medium to change aconverging position of the light beam, spherical aberration changingmeans for changing spherical aberration occurring on a convergingposition of the light beam converged by the converging means, a secondactuator for moving the spherical aberration changing means in arelatively precise manner, a third actuator for moving the sphericalaberration changing means in a relatively rough manner, light-receivingmeans for receiving light reflected from the information storage mediumof the light beam, converging state detecting means for detecting asignal according to a converging state on the information layer of theinformation storage medium of the light beam based on a signal of thelight-receiving means, focus control means for driving the firstactuator based on a signal of the converging state detecting means andperforming control so that the light beam is converged on a desiredposition of the information layer of the information storage medium,interlayer moving means for driving the first actuator so as to move theconverging position of the light beam to another information layer,spherical aberration detecting means for detecting a signal, based on asignal of the light-receiving means, according to an amount of sphericalaberration occurring on the converging position of the light beam on theinformation layer of the information storage medium, and sphericalaberration control means for driving the second actuator and the thirdactuator based on a signal of the spherical aberration detecting meansand performing control so that spherical aberration is almost 0, whereinthe third actuator moves the spherical aberration changing means atleast based on a direct current component included in the signal of thespherical aberration detecting means, the second actuator moves thespherical aberration changing means based on an alternating currentcomponent included in the signal of the spherical aberration detectingmeans, and when the converging position of the light beam is moved toanother information layer by the interlayer moving means, the sphericalaberration changing means is driven by the third actuator so as tominimize spherical aberration caused by the movement.

[0046] In a preferred embodiment, a signal based on an amount ofspherical aberration occurring on another information layer is appliedto the third actuator as an offset when the converging position of thelight beam is moved to another information layer by the interlayermoving means.

[0047] In a preferred embodiment, an operation of the sphericalaberration control means based on the signal of the spherical aberrationdetecting means is not performed until the converging position of thelight beam is moved to another information layer by the interlayermoving means and the signal of the converging state detecting means isconverged within a predetermined range.

[0048] According to still another aspect of the invention, an opticaldisc device is provided which comprises: an optical head for storing, asone unit, light beam emitting means for emitting a light beam,converging means for converging the light beam toward an informationstorage medium, a first actuator for moving the converging meanssubstantially perpendicularly to an information layer of the informationstorage medium to change a converging position of the light beam,spherical aberration changing means for changing spherical aberrationoccurring on a converging position of the light beam converged by theconverging means, a second actuator for moving the spherical aberrationchanging means, a third actuator for moving the spherical aberrationchanging means, and light-receiving means for receiving light reflectedfrom the information storage medium of the light beam, converging statedetecting means for detecting a signal according to a converging stateon the information layer of the information storage medium of the lightbeam based on a signal of the light-receiving means, focus control meansfor driving the first actuator based on a signal of the converging statedetecting means and performing control so that the light beam isconverged on a desired position of the information layer of theinformation storage medium, spherical aberration detecting means fordetecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, spherical aberration control means fordriving the second actuator and the third actuator based on a signal ofthe spherical aberration detecting means and performing control so thatspherical aberration is almost 0, and searching means for moving theoptical head in a radius direction of the information storage medium,wherein the third actuator moves the spherical aberration changing meansat least based on a direct current component included in the signal ofthe spherical aberration detecting means, the second actuator moves thespherical aberration changing means based on an alternating currentcomponent included in the signal of the spherical aberration detectingmeans, and when the converging position of the light beam is moved to adifferent radius position of the information storage medium by thesearching means, the third actuator is driven so as to minimizespherical aberration caused by the movement.

[0049] In a preferred embodiment, a signal based on an amount ofspherical aberration occurring on a radius position of anotherinformation layer is applied to the third actuator as an offset when theconverging position of the light beam is moved to a radius position ofanother information layer by the searching means.

[0050] In a preferred embodiment, an operation of the sphericalaberration control means based on the signal of the spherical aberrationdetecting means is not performed until the converging position of thelight beam is moved to a radius position of another information layer bythe searching means and the signal of the converging state detectingmeans is converged within a predetermined range on the radius positionof another information layer.

[0051] According to another aspect of the invention, an optical discdevice for performing recording and reproduction on an informationstorage medium having at least two information layers in a laminatedstructure is provided, which is characterized by comprising: light beamemitting means for emitting a light beam, converging means forconverging the light beam toward the information storage medium, a focusactuator for moving the converging means substantially perpendicularlyto the information layer of the information storage medium to change aconverging position of the light beam, light-receiving means forreceiving light reflected from the information storage medium of thelight beam, converging state detecting means for detecting a signalaccording to a converging state on the information layer of theinformation storage medium of the light beam based on a signal of thelight-receiving means, focus control means for driving the focusactuator based on a signal of the converging state detecting means andperforming control so that the light beam is converged on a desiredposition of the information layer of the information storage medium,spherical aberration detecting means for detecting a signal, based on asignal of the light-receiving means, according to an amount of sphericalaberration occurring on the converging position of the light beam on theinformation layer of the information storage medium, sphericalaberration changing means for changing spherical aberration occurring onthe converging position of the light beam converged by the convergingmeans, the change being made by driving with an elastic body, sphericalaberration control means for driving the spherical aberration changingmeans based on a signal of the spherical aberration detecting means andperforming control so that spherical aberration is almost 0, offsetapplying means for applying an offset to the spherical aberrationchanging means, and offset switching means for switching an offsetamount of the offset applying means according to the information layerof the information storage medium.

[0052] In a preferred embodiment, when the spherical aberration controlmeans is not operated, a predetermined offset is applied to thespherical aberration changing means by the offset applying means, andwhen the spherical aberration control means is operated, an offset isdetermined based on an average of driving output of the sphericalaberration changing means for a circumference of the information storagemedium and the offset of the offset applying means is switched.

[0053] According to another aspect of the invention, an optical discdevice is provided which comprises: light beam emitting means foremitting a light beam, converging means for converging the light beamtoward an information storage medium, a focus actuator for moving theconverging means substantially perpendicularly to the information layerof the information storage medium to change a converging position of thelight beam, spherical aberration changing means for changing sphericalaberration occurring on the converging position of the light beamconverged by the converging means, light-receiving means for receivinglight reflected from the information storage medium of the light beam,converging state detecting means for detecting a signal according to aconverging state on the information layer of the information storagemedium of the light beam based on a signal of the light-receiving means,focus control means for driving the focus actuator based on a signal ofthe converging state detecting means and performing control so that thelight beam is converged on a desired position of the information layerof the information storage medium, spherical aberration detecting meansfor detecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, spherical aberration control means formoving the spherical aberration changing means based on a signal of thespherical aberration detecting means and performing control so thatspherical aberration is almost 0, and dead band area generating meansfor preventing a signal of the spherical aberration control means frombeing transmitted to the spherical aberration changing means when thesignal of the spherical aberration control means has a value within apredetermined range.

[0054] According to still another aspect of the invention, an opticaldisc device is provide which comprises: converging means for converginga light beam toward an information storage medium, a focus actuator formoving the converging means substantially perpendicularly to aninformation layer of the information storage medium, sphericalaberration changing means for changing spherical aberration occurring ona converging position of the light beam converged by the convergingmeans, driving means for operating the spherical aberration changingmeans, light-receiving means for receiving light reflected from theinformation storage medium of the light beam, converging state detectingmeans for detecting a signal according to a converging state on theinformation layer of the information storage medium of the light beambased on a signal of the light-receiving means, focus control means fordriving the focus actuator based on a signal of the converging statedetecting means and performing control so that the light beam isconverged on a desired position of the information layer of theinformation storage medium, spherical aberration detecting means fordetecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, spherical aberration control means fordriving the driving means based on a signal of the spherical aberrationdetecting means and performing control so that spherical aberration isalmost 0, and spherical aberration signal correcting means foramplifying a signal of the converging state detecting means by apredetermined gain and then adding the signal to a detection signal ofthe spherical aberration detecting means. In a preferred embodiment, theoptical disc device further comprises: first test signal generatingmeans for applying a test signal to the focus actuator, first amplitudedetecting means for detecting amplitude of the detection signal of thespherical aberration detecting means, and spherical aberrationcorrection learning means for calculating an added gain of the sphericalaberration signal correcting means so that the first amplitude detectingmeans detects minimum amplitude of the spherical aberration detectingsignal in a state in which the test signal is applied to the focusactuator by the first test signal generating means.

[0055] In a preferred embodiment, the spherical aberration correctionlearning means learns an added gain in a state in which the focuscontrol means is operated and the spherical aberration control means isnot operated.

[0056] In a preferred embodiment, the spherical aberration signalcorrection means comprises added gain storing means for storing an addedgain for each layer in the information unit having information layers ina laminated structure, and added gain switching means for retrieving anadded gain corresponding to a position of the optical beam from theadded gain storing means and switching the added gain.

[0057] In a preferred embodiment, the optical disc device furthercomprises: first test signal generating means for applying a test signalto the focus actuator, focus control gain adjusting means for adjustinga gain of the focus control means, second test signal generating meansfor applying a test signal to the driving means, and sphericalaberration control gain adjusting means for adjusting a gain of thespherical aberration control means, wherein when the focus control meansand the spherical aberration control means are operated, the focuscontrol gain adjusting means makes an adjustment based on a first testsignal generated by the first test signal generating means and the firsttest signal after focus control, and the spherical aberration controlgain adjusting means makes an adjustment based on a spherical aberrationtest signal generated by the second test signal generating means and thespherical aberration test signal after spherical aberration control.

[0058] According to still another aspect of the invention, an opticaldisc device is provided which comprises: converging means for converginga light beam toward an information storage medium, a focus actuator formoving the converging means substantially perpendicularly to aninformation layer of the information storage medium, sphericalaberration changing means for changing spherical aberration occurring ona converging position of the light beam converged by the convergingmeans, driving means for operating the spherical aberration changingmeans, light-receiving means for receiving light reflected from theinformation storage medium of the light beam, converging state detectingmeans for detecting a signal according to a converging state on theinformation layer of the information storage medium of the light beambased on a signal of the light-receiving means, focus control means fordriving the focus actuator based on a signal of the converging statedetecting means and performing control so that the light beam isconverged on a desired position of the information layer of theinformation storage medium, spherical aberration detecting means fordetecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, spherical aberration control means fordriving the driving means based on a signal of the spherical aberrationdetecting means and performing control so that spherical aberration isalmost 0, and converging state detection signal correcting means foramplifying the signal of the spherical aberration detecting means by apredetermined gain and then adding the signal to the detection signal ofthe converging state detecting means.

[0059] In a preferred embodiment, the optical disc further comprisesfocus control means which does not add the detection signal of thespherical aberration detecting means to the detection signal of theconverging state detecting means, the detection signal of the sphericalaberration detecting means having been multiplied by a predeterminedmultiple by the converging state detection signal correcting means,which drives the focus actuator only based on the detection signal ofthe converging state detecting means, and performs control so that thelight beam is converged on a converging position of the informationlayer of the information storage medium when the spherical aberrationcontrol means is not performed.

[0060] In a preferred embodiment, the optical disc device furthercomprises: second test signal generating means for applying a testsignal to the driving means, and second amplification detecting meansfor detecting amplitude of the detection signal of the converging statedetecting means, converging state detection correction learning meansfor calculating an added gain of the converging state detection signalcorrecting means so that an effective value of the converging statedetection signal is minimized by the second amplitude detecting means ina state in which the test signal is applied to the driving means by thesecond test signal generating means.

[0061] In a preferred embodiment, the converging state detectioncorrection learning means is operated by the focus control means andlearns an added gain in a state in which the spherical aberrationcontrol means is not operated.

[0062] In a preferred embodiment, the optical disc device furthercomprises: first test signal generating means for applying a test signalto the focus actuator, focus control gain adjusting means for adjustinga gain of the focus control means, second test signal generating meansfor applying a test signal to the driving means, and sphericalaberration control gain adjusting means for adjusting a gain of thespherical aberration control means, wherein when the focus control meansand the spherical aberration control means are operated, the focuscontrol gain adjusting means makes an adjustment based on a first testsignal generated by the first test signal generating means and the firsttest signal after focus control, and the spherical aberration controlgain adjusting means makes an adjustment based on a spherical aberrationtest signal generated by the second test signal generating means and thespherical aberration test signal after spherical aberration control.

[0063] According to still another aspect of the invention, an opticaldisc device Is provided which comprises: converging means for converginga light beam toward an information storage medium, a focus actuator formoving the converging means substantially perpendicularly to aninformation layer of the information storage medium, sphericalaberration changing means for changing spherical aberration occurring ona converging position of the light beam converged by the convergingmeans, driving means for operating the spherical aberration changingmeans, light-receiving means for receiving light reflected from theinformation storage medium of the light beam, converging state detectingmeans for detecting a signal according to a converging state on theinformation layer of the information storage medium of the light beambased on a signal of the light-receiving means, focus control means fordriving the focus actuator based on a signal of the converging statedetecting means and performing control so that the light beam isconverged on a desired position of the information layer of theinformation storage medium, spherical aberration detecting means fordetecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, lowpass filter means for retrieving acomponent lower than a predetermined frequency of an output signal ofthe spherical aberration detecting means, spherical aberration controlmeans for driving the driving means based on a signal of the lowpassfilter means and performing control so that spherical aberration isalmost 0, highpass filter means for retrieving a component higher thanthe predetermined frequency of the output signal of the sphericalaberration detecting means, and spherical aberration signal adding meansfor adding a signal of the highpass filter means to the signal of theconverging state detecting means.

[0064] According to still another aspect of the invention, an opticaldisc device is provided which comprises: converging means for converginga light beam toward an information storage medium, a focus actuator formoving the converging means substantially perpendicularly to aninformation layer of the information storage medium, sphericalaberration changing means for changing spherical aberration occurring ona converging position of the light beam converged by the convergingmeans, driving means for operating the spherical aberration changingmeans, light-receiving means for receiving light reflected from theinformation storage medium of the light beam, converging state detectingmeans for detecting a signal according to a converging state on theinformation layer of the information storage medium of the light beambased on a signal of the light-receiving means, focus control means fordriving the focus actuator based on a signal of the converging statedetecting means and performing control so that the light beam isconverged on a desired position of the information layer of theinformation storage medium, spherical aberration detecting means fordetecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, and spherical aberration control means fordriving the driving means based on a detection signal of the sphericalaberration detecting means and performing control so that sphericalaberration is almost 0, wherein the focus control means has a band tentimes larger than a band of the spherical aberration control means.

[0065] According to still another aspect of the invention, an opticaldisc device is provided which comprises: converging means for converginga light beam toward an information storage medium having a spiral or aconcentric track, a focus actuator for moving the converging meanssubstantially perpendicularly to an information layer of the informationstorage medium, spherical aberration changing means for changingspherical aberration occurring on a converging position of the lightbeam converged by the converging means, driving means for operating thespherical aberration changing means, a tracking actuator for moving theconverging means in a direction of crossing the track on the informationstorage medium, light-receiving means for receiving light reflected fromthe information storage medium of the light beam, converging statedetecting means for detecting a signal according to a converging stateon the information layer of the information storage medium of the lightbeam based on a signal of the light-receiving means, focus control meansfor driving the focus actuator based on a signal of the converging statedetecting means and performing control so that the light beam isconverged on a desired position of the information layer of theinformation storage medium, spherical aberration detecting means fordetecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, spherical aberration control means fordriving the driving means based on a signal of the spherical aberrationdetecting means and performing control so that spherical aberration isalmost 0, track displacement detecting means for detecting a signalcorresponding to a displacement of the light beam relative to the trackof the information storage medium based on the signal of thelight-receiving means, tracking control means for driving the trackingactuator based on a signal of the track displacement detecting means andperforming control so that the light beam scans the track, transfermeans for permitting the tracking actuator to move in a radius directionof an information unit, and transfer driving means for driving thetransfer means, wherein when the transfer means is operated in a statein which the focus control means is operated and the tracking controlmeans is not operated, the spherical aberration changing means is movedby a predetermined amount.

BRIEF DESCRIPTION OF DRAWINGS

[0066]FIG. 1 is a schematic diagram showing an optical disc.

[0067]FIG. 2 is a schematic diagram having an enlarger information layerof the optical disc.

[0068]FIG. 3 is a block diagram showing the configuration of aconventional optical disc device.

[0069]FIG. 4 is a block diagram showing the configuration of alight-receiving part and a preamplifier in the conventional optical discdevice.

[0070]FIG. 5 is a schematic diagram showing an optical disc having aplurality of information layers.

[0071] FIGS. 6(a) and 6(b) are waveform charts showing the drivingsignals of a focus during the interlayer movement of the conventionaloptical disc device.

[0072]FIG. 7 is a sectional view showing a spherical aberrationcorrection lens.

[0073]FIG. 8 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 1 of the present invention.

[0074]FIG. 9 is a sectional view showing a light beam to explain amethod of detecting spherical aberration.

[0075]FIG. 10 is a sectional view showing the configuration of thelight-receiving part in detail.

[0076]FIG. 11 is a block diagram showing the detail of thelight-receiving part and the preamplifier.

[0077] FIGS. 12(a) to 12(c) are waveform charts showing driving signalfor spherical aberration correction according to Embodiment 1.

[0078]FIG. 13 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 2 of the present invention.

[0079] FIGS. 14(a) to 14(d) are waveform charts showing driving signalfor spherical aberration correction during interlayer movement accordingto Embodiment 2.

[0080]FIG. 15 is a flowchart showing the sequence of sphericalaberration correction during interlayer movement according to Embodiment2.

[0081]FIG. 16 is a waveform chart showing signals and the positions of aconverging lens and information layers L0 and L1 during interlayermovement according to Embodiment 2.

[0082]FIG. 17 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 3 of the present invention.

[0083] FIGS. 18(a) to 18(d) are waveform charts showing driving signalsfor spherical aberration correction during movement in the radiusdirection according to Embodiment 3.

[0084]FIG. 19 is a flowchart showing the sequence of sphericalaberration correction during movement in the radius direction accordingto Embodiment 3.

[0085]FIG. 20 is a waveform chart showing signals, a converging lens,and a change in the pressure of the substrate in a disc during movementin the radius direction according to Embodiment 3.

[0086]FIG. 21 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 4 of the present invention.

[0087] FIGS. 22(a) to 22(d) are waveform charts showing driving signalsfor spherical aberration correction during interlayer movement accordingto Embodiment 4.

[0088]FIG. 23 is a flowchart showing the sequence of sphericalaberration correction during interlayer movement according to Embodiment4.

[0089]FIG. 24 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 5 of the present invention.

[0090] FIGS. 25(a) to 25(d) are waveform charts showing driving signalsfor spherical aberration correction during interlayer movement accordingto Embodiment 5.

[0091]FIG. 26 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 6 of the present invention.

[0092] FIGS. 27(a) and 27(b) are sectional views showing light beams toexplain a method of detecting spherical aberration.

[0093] FIGS. 28(a) to 28(e) are waveform charts for explaining thecorrection of a spherical aberration detection signal according toEmbodiment 6.

[0094]FIG. 29 is a block diagram showing the configuration of an opticaldisc device to explain a method of learning an amplification factor of aspherical aberration signal correcting section according to Embodiment6.

[0095] FIGS. 30(a) to 30(g) are waveform charts for explaining thelearning of a spherical aberration signal correcting section accordingto Embodiment 6.

[0096]FIG. 31 is a flowchart showing the learning sequence of thespherical aberration signal correcting section according to Embodiment6.

[0097] FIGS. 32(a) to 32(f) are waveform charts showing the switching ofan amplification factor of the spherical aberration signal correctingsection during interlayer movement according to Embodiment 6.

[0098]FIG. 33 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 7 of the present invention.

[0099] FIGS. 34(a) to 34(g) are waveform charts for explaining thecorrection of an FE signal according to Embodiment 7.

[0100]FIG. 35 is a block diagram showing an FE signal correcting section30 of Embodiment 7.

[0101] FIGS. 36(a) to 36(g) are waveform charts for explaining thelearning of an FE signal correcting section according to Embodiment 7.

[0102]FIG. 37 is a flowchart showing the learning sequence of the FEsignal correcting section according to Embodiment 7.

[0103]FIG. 38 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 8 of the present invention.

[0104]FIG. 39 is a characteristic diagram for explaining jitter relativeto spherical aberration and focus offset.

[0105] FIGS. 40(a) to 40(d) are waveform charts for explaining a methodof correcting the influence of remaining spherical aberration bydefocusing according to Embodiment 8.

[0106]FIG. 41 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 9 of the present invention.

[0107]FIGS. 42A to 42D are characteristic diagrams for explaining acontrol band and the influence of interference according to Embodiment9.

[0108]FIG. 43 is a block diagram for explaining the control band and theinfluence of interference according to Embodiment 9.

[0109]FIGS. 44A to 44D are characteristic diagrams for explaining thecharacteristics of a control section, a driving circuit, and an actuatoraccording to Embodiment 9.

[0110]FIG. 45 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 10 of the present invention.

[0111] FIGS. 46(a) to 46(e) are waveform charts for explaining sphericalaberration correction during a search according to Embodiment 10.

[0112]FIG. 47 is a flowchart showing the sequence of sphericalaberration correction during movement in the radius direction accordingto Embodiment 10.

[0113] FIGS. 48(a) to 48(c) are waveform charts showing the influence ofthe crossing of grooves upon a focus error signal according toEmbodiment 10.

[0114] FIGS. 49(a) to 49(e) are waveform charts showing the influence ofdefocus upon a spherical aberration detection signal.

[0115] FIGS. 50(a) to 50(e) are waveform charts showing the influence ofdefocus upon the spherical aberration detection signal.

[0116] FIGS. 51(a) to 51(e) are waveform charts showing the influence ofdifferent information layers upon the spherical aberration detectionsignal.

[0117] FIGS. 52(a) to 52(e) are waveform charts showing the influence ofdifferent information layers upon the spherical aberration detectionsignal.

[0118] FIGS. 53(a) to 53(c) are schematic diagrams showing the influenceof the position of the spherical aberration correction lens upon adistance from an objective lens to a focus, and

[0119] FIGS. 54(a) to 54(c) are schematic diagrams showing the influenceof the position of the spherical aberration correction lens upon adistance from the objective lens to the focus.

BEST MODE FOR CARRYING OUT THE INVENTION

[0120] The embodiments of the present invention will be described below.

[0121] (Embodiment 1)

[0122]FIG. 8 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 1 of the present invention. FIG. 9is a sectional view showing a light beam to describe a method ofdetecting spherical aberration in the present embodiment. FIG. 10 is asectional view particularly showing the detail of a light-receiving part37 in the optical disc device of FIG. 8. FIG. 11 is a block diagramshowing the detail of the light-receiving part 37 and a preamplifier 12in the optical disc device of FIG. 8. In these drawings, componentscorresponding to the constituent elements of a conventional optical discdevice are indicated by the same reference numerals.

[0123] As with the optical disc device of FIG. 3, focus control in thepresent embodiment is performed by driving an objective lens 1 by afocus actuator 2 serving as a first actuator.

[0124] In the spherical aberration correction of the present embodiment,a spherical aberration correction lens unit 15 is used which acts asspherical aberration changing means, and the correction is performed bytwo kinds of actuators (first and second actuators) 34 and 35 fordriving the spherical aberration correction lens unit 15. This pointwill be described in detail below.

[0125] In the present embodiment, the spherical aberration correctionlens unit 15 of FIG. 7 is provided. The spherical aberration correctionlens unit 15 comprises a spherical aberration correction actuator(second actuator) 34 for finely moving one of a pair of lensesconstituting the spherical aberration correction lens unit 15, and astepping motor 35 (third actuator) for roughly moving the sphericalaberration correction lens unit 15 and the spherical aberrationcorrection actuator 34.

[0126] The spherical aberration correction actuator 34 acting as asecond actuator is provided to drive the spherical aberration correctionlens unit 15 acting as spherical aberration changing means. Thespherical aberration correction actuator 34 changes a lens interval ofthe spherical aberration correction lenses 15 to adjust sphericalaberration. A range for permitting the spherical aberration correctionactuator 34 to move one of the spherical aberration correction lenses15, that is, a movable distance, is smaller than that of the steppingmotor 35 (described later) acting as a third actuator. However, thespherical aberration correction actuator 34 precisely responds to asignal (particularly a signal of a high frequency) corresponding to analternating current component (AC component) included in a sphericalaberration correction signal, which is calculated from a sphericalaberration detection signal. Thus the actuator 34 moves the sphericalaberration correction lens unit 15 to correct spherical aberration.

[0127] The stepping motor 35 serving as a third actuator moves one ofthe spherical aberration correction lenses 15 and the sphericalaberration correction actuator 34. The stepping motor 35 is less capableof following a high-frequency signal but has a wider range for movingthe spherical aberration correction lens unit 15 (movable distance) ascompared with the spherical aberration correction actuator 34. Hence,the stepping motor 35 can smoothly follow a DC signal and alow-frequency signal.

[0128] In the present embodiment, the stepping motor 35 moves thespherical aberration correction lens unit 15 to roughly correctspherical aberration in response to a signal of a direct currentcomponent (DC component) included in a spherical aberration correctionsignal calculated based on a signal (spherical aberration detectionsignal) from a spherical aberration detector 31. A precisecorrection ofspherical aberration is carried out by the spherical aberrationcorrection actuator 34 serving as a second actuator.

[0129] The spherical aberration correction actuator 34 and the steppingmotor 35 are driven by a beam expander precise driving circuit 33 and abeam expander rough driving circuit 32, respectively. The beam expanderprecise driving circuit 33 and the beam expander rough driving circuit32 each amplify an AC component and a DC component of a control signal(spherical aberration correction signal) outputted from themicrocomputer 8. The spherical aberration correction signal is outputtedfrom the microcomputer 8 based on the spherical aberration detectionsignal.

[0130] Referring to FIGS. 8 to 12, the spherical aberration correctioncontrol of Embodiment 1 will be described in detail below. FIG. 12 is awaveform chart showing driving signals for spherical aberrationcorrection according to Embodiment 1.

[0131] Firstly referring to FIG. 8, a focus error signal generator 36acting as converging state detecting means detects a signalcorresponding to a converging state of a light beam on the informationlayer of the optical disc 20, based on a signal from the light-receivingpart 37 serving as light-receiving means. To be specific, the focuserror signal generator 36 generates a signal indicating a radialposition error between a light beam spot, which is outputted from theoptical head 5 and is focused, and the optical disc 20.

[0132] A method of generating a focus error signal (hereinafter,referred to as an FE signal) will be discussed in detail below. As shownin FIG. 10, the light-receiving part 37 divides, by using a polarizedbeam splitter 47, a light beam passing through a detection lens 46. Afirst light shielding plate 48 passes only an outer peripheral lightbeam and a second light shielding plate 49 passes only an innerperipheral light beam. Light quantities of the light beams are detectedby an outer peripheral light-receiving part 40 and an inner peripherallight-receiving part 41, respectively.

[0133] As shown in FIG. 11, the outer peripheral light-receiving part 40and the inner peripheral light-receiving part 41 are each divided intofour areas A, B, C, and D. Each of the areas generates photocurrentaccording to a detected light quantity and outputs the photocurrent tocorresponding I/V converters 42 a to 42 d and I/V converters 43 a to 43d that are included in the preamplifier 12.

[0134] Signals converted from current to voltage by the I/V converters42 a to 42 d and the I/V converters 43 a to 43 d are subjected tooperations similar to those of a conventional focus error signalgenerator 7 in an outer peripheral focus error signal generator 44 andan inner peripheral focus error signal generator 45, so that the signalsare converted into an outer peripheral focus error signal and an innerperipheral focus error signal.

[0135] A focus error signal actually used for focus control inEmbodiment 1 is obtained by adding the outer peripheral focus errorsignal and the inner peripheral focus error signal in a focus errorsignal generator 36.

[0136] In this way, the method of generating the focus error signalaccording to the present embodiment is somewhat different from aconventional method of generating a focus error signal according to theastigmatic method. However, the characteristics are the same. Hence, byusing the FE signal serving as an output signal of the focus errorsignal generator 36, a light beam spot is driven so as to have apredetermined converging state on the information recording surface 20of the optical disc 20 in the same may as a conventional device and thusfocus control is achieved.

[0137] Referring to FIGS. 9, 11, and 12, the following will describe amethod of detecting a spherical aberration detection signal and acontrolling method using the same.

[0138] In a state in which focus control is performed, a light beamemitted from the optical head 5 is refracted by a substrate 21 of theoptical disc 20 as shown in FIG. 2, and an outer peripheral light beamis focused on a focal point B and an inner peripheral light beam isfocused on a focal point C.

[0139] When spherical aberration does not occur on the informationrecording surface of the optical disc 20, the focal point B of the outerperipheral light beam and the focal point C of the inner peripherallight beam are both coincident with a focal point A. However, as theinfluence of spherical aberration increases, the focal point B and thefocal point C are separated from each other, and the two focuses areboth placed in a defocus state with respect to the information recordingsurface where a light beam should converge.

[0140] As shown in FIG. 11, a spherical aberration detector 31 servingas spherical aberration detecting means detects an influence amount ofspherical aberration on the outer peripheral light beam (a defocusamount on the focal point B) and an influence amount of sphericalaberration on the inner peripheral light beam (a defocus amount on thefocal point C). Then, the spherical aberration detector 31 detects asignal according to an amount of spherical aberration occurring on theconverging position of the light beam. To be specific, a difference iscalculated between the outer peripheral focus error signal and the innerperipheral focus error signal, which are the output signals of the outerperipheral focus error signal generator 44 and the inner peripheralfocus error signal generator 45, so that a spherical aberrationdetection signal is generated according to an amount of sphericalaberration occurring on the converging position of the light beam.

[0141] The spherical aberration detection signal serving as an outputsignal of the spherical aberration detector 31 is inputted to themicrocomputer 8, a filtering operation such as phase compensation andgain compensation is performed on the signal, and thus a sphericalaberration correction signal for correcting spherical aberration isgenerated. The microcomputer 8, which is focus control means and acts asspherical aberration control means, performs frequency separation on thespherical aberration correction signal after the filtering operation.The beam expander rough driving circuit 32 responding to a DC componentof the spherical aberration correction signal transmits to the steppingmotor a driving signal for moving the spherical aberration correctionlens unit 15 to a position where the spherical aberration correctionsignal has a DC component of almost 0 (see FIG. 12(b)). The steppingmotor 35 having received the driving signal moves the sphericalaberration correction lens unit 15 (time t1) and performs correction sothat the DC component of spherical aberration is almost 0.

[0142] Then, the microcomputer 8 outputs to the beam expander precisedriving circuit 33 a driving signal for moving the spherical aberrationcorrection lens unit 15 to a position where an AC component included ina spherical aberration correction signal, which cannot be corrected bythe stepping motor 35, is almost 0 as shown in FIG. 12(c) (time t2). Thespherical aberration correction actuator 34 having received the drivingsignal moves the spherical aberration correction lens unit 15 andperforms correction control so that spherical aberration is almost 0,that is, the focal point B and the focal point C are coincident witheach other (in other words, the focal point B and the focal point C areboth close to the focal point A).

[0143] To be specific, a filtering operation is performed on thespherical aberration detection signal, which is an output signal of thespherical aberration detector 31, by the microcomputer 8. With the DCcomponent of the spherical aberration detection signal after a filteringoperation, correction control is performed so that the focuses A, B, andC are made coincident with one another by the spherical aberrationcorrection lens unit 15 driven by the beam expander rough drivingcircuit 32 and the stepping motor 35. Further, with the AC component,correction control is performed so that the focuses A, B, and C are madecoincident with one another by the spherical aberration correction lensunit 15 driven by the beam expander precise driving circuit 33 and thespherical aberration correction actuator 34.

[0144] In the present embodiment, regarding the DC component of thespherical aberration correction signal, the beam expander rough drivingcircuit 32 transmits a driving signal for setting the DC component atalmost 0 to the stepping motor 35, so that the stepping motor 35 movesthe spherical aberration correction lens unit 15 and sphericalaberration correction is performed for the DC component. Regarding theAC component of the spherical aberration correction signal, the beamexpander precise driving circuit 33 transmits a driving signal forsetting the AC component at almost 0 to the spherical aberrationcorrection actuator 34, so that the spherical aberration correctionactuator 34 moves the spherical aberration correction lens unit 15 andspherical aberration correction is performed for the AC component. Thus,even when an objective lens with an NA larger than that of theconventional objective lens (e.g., an NA of 0.8 or higher and 0.85 orhigher) is used to increase the recording density of the optical disc20, high responsivity is achieved and spherical aberration correction iscontrolled over a wide range.

[0145] Further, in the control of the stepping motor 35 based on thebeam expander rough driving signal, the spherical aberration correctionsignal of the AC component lower than the rotational frequency of theoptical disc 20 and the spherical aberration correction signal of the DCsignal are transmitted to the beam expander rough driving circuit 32,and the spherical aberration correction signal of the AC componenthigher than the rotational frequency of the optical disc 20 istransmitted to the beam expander precise driving circuit 33. With thisconfiguration, the stepping motor 35 with a low tracking speed canfollow a change in thickness along the radius direction of the substratewithout causing transient response due to the influence of the uneventhickness of the substrate 21 for one rotation, increase accuracy forcontrolling spherical aberration correction, and further improveresponsivity for spherical aberration correction.

[0146] (Embodiment 2)

[0147]FIG. 13 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 2. FIG. 14 is a waveform chartshowing driving signals for correcting spherical aberration during theinterlayer movement of the present embodiment. FIG. 15 is a flowchartshowing the sequence of spherical aberration correction during theinterlayer movement of the present embodiment. In these drawings, thesame members and components as those of the conventional art andEmbodiment 1 are indicated by the same reference numerals and theexplanation thereof is omitted.

[0148] A microcomputer 8 and a focus actuator driving circuit 9constitute interlayer moving means for driving a focus actuator. In FIG.13, a driving position selecting section 13 retrieves a target drivingposition from a driving position storing section 14 and outputs thedriving position to a beam expander rough driving circuit 32.

[0149] Further, as Embodiment 1, focus control and spherical aberrationcontrol are performed based on a focus error signal, which is an addedsignal of an outer peripheral focus error signal and an inner peripheralfocus error signal, and a spherical aberration detection signal, whichis a difference signal of the outer peripheral focus error signal andthe inner peripheral focus error signal.

[0150] Referring to FIGS. 13 to 15, the following will describespherical aberration correction control during the interlayer movementof Embodiment 2 configured thus.

[0151] As shown in FIGS. 14(c) and 14(d), during the interlayermovement, the microcomputer 8 firstly stops an output to a beam expanderprecise driving circuit 33 at time t1, the output being based on anoutput signal from a spherical aberration detector 31, and themicrocomputer 8 stops an output to the focus actuator driving circuit 9,the output corresponding to an output from a focus error signalgenerator 36, so that correction control and focus control for sphericalaberration are made inoperative, that is, the control is stopped (S1 andS2 of FIG. 15).

[0152] Subsequently, as shown in FIG. 14(d), a driving command forinterlayer movement is outputted to the focus actuator driving circuit 9until time t2 according to steps similar to those of a conventional art(S3 of FIG. 15). When a driving command for interlayer movement iscompleted at the time t2, the microcomputer 8 simultaneously cancels thestop of the output to the focus actuator driving circuit 9, the outputbeing based on an output from the focus error signal generator 36. Thenas shown in FIG. 14(d), the microcomputer 8 resumes focus control (S4 ofFIG. 15).

[0153] Subsequently, after waiting for stable focus control until timet3 (S5 of FIG. 15), the microcomputer 8 retrieves information about thedriving position of a spherical aberration correction lens unit 15, theposition being suitable for an information recording surface at adestination, from a driving position storing section 14 which stores theinformation and is shown in FIG. 13 by using a driving positionselecting section. As shown in FIG. 14(b), the microcomputer 8 outputsto the stepping motor 35 a driving signal (offset signal) for moving thespherical aberration correction lens unit 15 to the driving positionwith respect to the beam expander coarse driving circuit 32. Thus, thestepping motor 35 is driven and a DC component of a spherical aberrationdetection signal is set at almost 0 as shown in FIG. 14(a) (S6 and S7 ofFIG. 15).

[0154] Finally, the microcomputer 8 cancels the stop of an output to thebeam expander precise driving circuit 33 at time t4, and it outputs acorrection signal (that is, the AC component of the spherical aberrationcorrection signal in the present embodiment) having not been correctedby the stepping motor 35 (S8 of FIG. 15) as is shown in FIG. 14(c). Thencorrection control for spherical aberration is resumed through aspherical aberration correction actuator 34.

[0155] Moreover, the timing of stopping focus control and sphericalaberration control and the timing of outputting a driving signal to thebeam expander rough driving circuit are set as below, so that fasteraccess can be made between layers.

[0156]FIG. 16 shows the positions of a converging lens and informationlayers L0 and L1 during the interlayer movement of a double-layer discand shows a waveform chart of signals. The following refers to FIG. 16.

[0157] It is assumed that a light beam scans a given track on theinformation layer L0. In this state, when data on the information layerL1 is reproduced, focus control and spherical aberration correctioncontrol are firstly made inoperative, that is, the control is stopped(time a). Then, after a driving command is issued to the focus actuatordriving circuit 9, information about a driving position for correctingspherical aberration is retrieved by the driving position selectingsection 13 from the driving position storing section 14, which storesthe information and is shown in FIG. 13, the driving position beingsuitable for another information layer serving as a target layer (theinformation layer L1 in the present embodiment). A driving signal formoving the spherical aberration correction lens unit 15 to the retrievedposition is outputted to the beam expander rough driving circuit 32(time b).

[0158] Hence, as the focus of the objective lens 1 is closer to theinformation layer L1 from the information layer L0, the stepping motor35 moves so as to minimize spherical aberration caused by the movement,that is, a spherical aberration correction amount becomes closer to thereference amount of the information layer L1. Thus, an FE signal and atotal quantity of light reflected from the optical disc 20 are lessaffected and the stability of focus jump is not interrupted due to alarge change in spherical aberration during focus jump. After themovement to the information layer L1, even when spherical aberrationcontrol is turned on immediately after focus control having beeninoperative is turned on (time c), spherical aberration control is notstabilized unless focus control is stabilized. For example, when an FEsignal is converged within a predetermined range during the observationof the FE signal, it is decided that focus control is stabilized andspherical aberration control having been inoperative is turned on (timed).

[0159] With this configuration, the stepping motor 35 (particularly thespherical aberration correction lens unit 15) is moved so as to reduce achange in spherical aberration occurring during interlayer movement.Thus, it is possible to stably switch spherical aberration control foreach layer with a great effect.

[0160] As described above, for a change in the DC component of sphericalaberration occurring during interlayer movement, correction is performedby using a rough driving system (stepping motor 35), so that sphericalaberration correction is controlled over a wide range not only on adouble-layer disc but also a multi-layer disc.

[0161] (Embodiment 3)

[0162]FIG. 17 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 3. FIG. 18 is a waveform chartshowing driving signals for spherical aberration control when movementis performed along the radius direction according to Embodiment 3. FIG.19 is a flowchart showing the sequence of spherical aberrationcorrection when movement is performed along the radius directionaccording to Embodiment 3. In these drawings, the same members andcomponents as those of the conventional art and Embodiment 1 areindicated by the same reference numerals and the explanation thereof isomitted.

[0163] In the present embodiment, an optical head 5 stores, as one unit,a light source 3 acting as light beam irradiating means for emitting alight beam, an objective lens 1 acting as converging means forconverging a light beam on an optical disc 20 serving as an informationstorage medium, a focus actuator 2 acting as a first actuator for movingthe objective lens 1 substantially perpendicularly to an informationlayer of the optical disc 20 in order to change the converging positionof a light beam, a spherical aberration correction lens unit 15 actingas spherical aberration changing means for changing spherical aberrationoccurring on the converging position of a light beam converged by theobjective lens 1, a stepping motor 35 for moving a spherical aberrationcorrection actuator 34, the spherical aberration correction actuator 34for moving the spherical aberration correction lens unit 15, and alight-receiving part 37 for receiving reflected light of a light beamfrom the optical disc 20.

[0164] The optical head 5 can be moved in the radius direction of theoptical disc 20 by a transfer table 60 acting as searching means.Moreover, the transfer table 60 is driven by an output signal (drivingsignal) from a transfer table driving circuit 62.

[0165] Further, as with Embodiment 1, focus control and sphericalaberration control are performed based on a focus error signal (a signaloutputted from a focus error signal generator 36), which is an addedsignal of an outer peripheral focus error signal and an inner peripheralfocus error signal, and a spherical aberration detection signal (asignal outputted from a spherical aberration detector 31), which is adifference signal of the outer peripheral focus error signal and theinner peripheral focus error signal.

[0166] Referring to FIGS. 17, 18, and 19, the following will describespherical aberration correction control during movement performed in theradius direction according to Embodiment 3 configured thus. As shown inFIG. 18(c), in the case of movement for retrieval and so on along theradius direction, the microcomputer 8 firstly stops an output to a beamexpander precise driving circuit 33, the output being based on an outputof the spherical aberration detector 31, at time t1 at which trackingcontrol is not performed, and the microcomputer 8 makes the sphericalaberration correction actuator 34 inoperative, so that correctioncontrol for spherical aberration is stopped (S1 of FIG. 19). As shown inFIG. 18(d), the microcomputer 8 outputs a transfer table driving signalto a transfer table driving circuit 62 until output time t2 (S2 of FIG.19).

[0167] The transfer table driving circuit 62 moves the transfer table60, which is loaded with the optical head 5, in the radius direction ofthe optical disc 20 based on the transfer table driving signaltransmitted from the microcomputer 8. Then, as shown in FIG. 18(b), themicrocomputer 8 outputs, to the beam expander rough driving circuit 32at time t3, a driving signal for setting the DC component of thespherical aberration detection signal at almost 0. The stepping motor 35is driven based on a driving signal transmitted from the beam expanderrough driving circuit 32, and the microcomputer 8 waits for the movementof the stepping motor 35 to a predetermined position (S3 and S4 of FIG.19).

[0168] Subsequently, the microcomputer 8 cancels the stop of an outputto the beam expander precise driving circuit 33 at time t4, the outputcorresponding to the output of a spherical aberration detector 31,outputs as shown in FIG. 18(c) a correction signal (that is, the ACcomponent of the spherical aberration correction signal) having not beencorrected by the stepping motor 35 to the beam expander precise drivingcircuit 33 (S5 of FIG. 19), and resumes correction control for sphericalaberration that have been made inoperative by the spherical aberrationactuator 34.

[0169] Moreover, the timing of stopping spherical aberration control andthe timing of outputting a driving signal to the beam expander roughdriving circuit 32 are set as below, so that faster access can be madein the radius direction.

[0170]FIG. 20 shows a change in the substrate pressure of the objectivelens 1 and the optical disc 20 during movement in the radius directionand shows a waveform chart of signals. The following refers to FIG. 20.It is assumed that a light beam scans a given track on the innerperiphery of the optical disc 20. In this state, when data on the outerperiphery is reproduced, the microcomputer 8 firstly makes inoperativetracking control and spherical aberration correction control, that is,stops the tracking control and the spherical aberration correctioncontrol (time a). Then, after a driving command is issued to thetransfer table driving circuit 62, the microcomputer 8 transmits aspherical aberration correction signal to the beam expander roughdriving circuit 32 in order to make a movement to a driving position ofthe spherical aberration correction lens unit 15, the driving positionbeing suitable for a substrate pressure on a target radius position, andthe beam expander rough driving circuit 32 outputs a driving signal(offset signal) corresponding to the transmitted spherical aberrationcorrection signal (time b).

[0171] Hence, as the transfer table 60 is closer to the outer peripheryfrom the inner periphery, the stepping motor 35 moves so as to minimizespherical aberration caused by the movement, that is, a sphericalaberration correction amount becomes closer to the reference amount onthe target outer peripheral position. Thus, it is possible to reduceinfluence upon an FE signal and a tracking error signal caused by alarge change in spherical aberration during movement in the radiusdirection, and the stability of a drawing operation of the trackingcontrol is not interrupted immediately after movement in the radiusdirection.

[0172] Even when the stop of tracking control (time c) and the stop ofspherical aberration control are successively cancelled and are turnedon after movement to the target outer peripheral position, if trackingcontrol is not stabilized, the tracking control may become moreunstable. Thus, for example when a tracking error signal is convergedwithin a predetermined range while a tracking error signal is observed,the microcomputer 8 decides that tracking control is stable, cancels thestop of spherical aberration control, and turns on the control (time d).With this configuration, it is possible to more stably switch sphericalaberration control for each radius during movement in the radiusdirection, achieving a great effect.

[0173] As described above, for a change in the DC component of sphericalaberration occurring during movement in the radius direction, correctionis performed by using a rough driving system (stepping motor 35), sothat spherical aberration correction is controlled over a wide rangeabsorbing an uneven thickness and uneven joining on the optical disc 20.

[0174] (Embodiment 4)

[0175]FIG. 21 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 4. FIG. 22 is a waveform chartshowing driving signals for correcting spherical aberration during theinterlayer movement of Embodiment 4. FIG. 23 is a flowchart showing thesequence of spherical aberration correction during the interlayermovement of Embodiment 4. In these drawings, the same members andcomponents as those of the conventional art and Embodiment 1 areindicated by the same reference numerals and the explanation thereof isomitted.

[0176] A microcomputer 8 comprises an offset amount storing section 68for storing an offset amount according to the information layers of anoptical disc 20 and an offset amount selecting section 67, which isoffset applying means and acts as offset switching means. By using theoffset amount selecting section 67, the microcomputer 8 retrieves, fromthe offset amount storing section 68, a desired storage valuecorresponding to each of the information layers of the optical disc 20,and the microcomputer 8 performs switching to the retrieved offsetamount. After the switched offset amount and a spherical aberrationcorrection signal are added by an adder 69, the result is used as adriving signal to a beam expander precise driving circuit 33, so that anoffset is applied to a spherical aberration correction lens unit 15.

[0177] A spherical aberration correction actuator 34 is driven by thebeam expander precise driving circuit 33 for performing currentamplification on control output from the microcomputer 8. An elasticbody such as a plate spring is mounted on the spherical aberrationcorrection lens unit 15 and force corresponding to a signal applied tothe spherical aberration correction actuator 34 is exerted to the platespring. As described above, force according to an offset amountcorresponding to each of the information layers is applied to the platespring for supporting the spherical aberration correction lens unit 15.Thus, the spherical aberration correction lens unit 15 can be movedfinely.

[0178] Further, as with Embodiment 1, a focus error signal is generatedfrom an added signal of an outer peripheral focus error signal and aninner peripheral focus error signal, and a spherical aberrationdetection signal is generated from a difference signal of the outerperipheral focus error signal and the inner peripheral focus errorsignal.

[0179] Referring to FIGS. 21 to 23, the following will describespherical aberration correction control during interlayer movementaccording to Embodiment 4 configured thus.

[0180] In the present embodiment, a spherical aberration detectionsignal outputted from a spherical aberration detector 31 is inputted tothe microcomputer 8 just like Embodiment 1 while focus control isperformed, and a filtering operation such as phase compensation and gaincompensation is performed in the microcomputer 8.

[0181] The microcomputer 8 selects, by using the offset amount selectingsection 67, an offset amount corresponding to the information layer at adestination from offset amounts stored in the offset amount storingsection 68, and the microcomputer 8 performs switching. Thereafter, themicrocomputer 8 adds, by using an adder 69, the switched offset amountand a spherical aberration correction signal obtained after thefiltering operation, and outputs the added signal to the beam expanderprecise driving circuit 33. The beam expander precise driving circuit 33performs correction control for spherical aberration based on aspherical aberration correction signal obtained after offset addition.

[0182] During interlayer movement, as shown in FIGS. 22(b) and 22(d),the microcomputer 8 firstly makes inoperative focus control andcorrection control for spherical aberration at time t1, that is, stopsthe focus control and correction control for spherical aberration (S1and S2 of FIG. 23). As shown in FIG. 22(d), the microcomputer 8 outputsa command to a focus actuator driving circuit 9 until t2 according tosteps similar to those of the conventional art (S3 of FIG. 23). Wheninterlayer movement similar to that of the conventional art iscompleted, focus control having been made inoperative is simultaneouslyresumed (S4 of FIG. 23). As shown in FIG. 22(c), the offset amountselecting section 67 of the microcomputer 8 simultaneously retrieves anoffset amount for the information recording surface at a destinationfrom the offset amount storing section 68 with respect to the beamexpander precise driving circuit 33. As shown in FIG. 22(b), the offsetamount is added to a beam expander precise driving signal.

[0183] Thus, the beam expander precise driving circuit 33 drives thespherical aberration correction actuator 34 based on the beam expanderprecise driving signal and set the DC component of the sphericalaberration detection signal at almost 0 (S5 of FIG. 23). After waitingfor stable focus control (S6 of FIG. 23), as shown in FIG. 22(b), themicrocomputer 8 outputs, to the beam expander precise driving circuit 33at time t3, a spherical aberration correction signal having not beencorrected only by an offset amount, cancels the stop of the sphericalaberration correction actuator 34, and resumes correction control forspherical aberration (S7 of FIG. 23).

[0184] As described above, regarding the DC component of sphericalaberration occurring during interlayer movement, an offset is added to aprecise driving system (spherical aberration actuator 34), achievingstable correction control for spherical aberration with high correctingaccuracy.

[0185] Further, the DC component of the spherical aberration detectionsignal is measured at predetermined time and an average value of the DCcomponent is added to an offset amount of the offset amount storingsection 68, the offset amount being currently selected by the offsetamount selecting section 67, so that the most proper target position isobtained for spherical aberration correction and tracking accuracy isfurther improved.

[0186] (Embodiment 5)

[0187]FIG. 24 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 5. FIGS. 25(a) to 25(d) are waveformcharts showing driving signals and so on for spherical aberrationcorrection during interlayer movement according to Embodiment 5. Inthese drawings, the same members and components as those of theconventional art and Embodiment 1 are indicated by the same referencenumerals and the explanation thereof is omitted.

[0188] As shown in FIG. 24, a microcomputer 8 of the present embodimentcomprises a dead band area generating section 70. The dead band areagenerating section 70 receives a signal outputted from a gain adjustingsection 66. The dead band area generating section 70 operates so thatthe signal is interrupted and is not transmitted to a beam expanderrough driving circuit 32 when the signal has an absolute value equal toor smaller than a predetermined value.

[0189] A stepping motor 35 is driven by the beam expander rough drivingcircuit 32 for performing current amplification on control output fromthe microcomputer 8.

[0190] A spherical aberration correction lens unit 15 can be moved bythe stepping motor 35 over a wide range. Further, as with Embodiment 1,a focus error signal is generated from an added signal of an outerperipheral focus error signal and an inner peripheral focus errorsignal, and a spherical aberration detection signal is generated from adifference signal of the outer peripheral focus error signal and theinner peripheral focus error signal.

[0191] Referring to FIGS. 24 and 25, the following will describecorrection control for spherical aberration according to Embodiment 5configured thus.

[0192] In the present embodiment, a spherical aberration detectionsignal outputted from a spherical aberration detector 31 is inputted tothe microcomputer 8 just like Embodiment 1 while focus control isperformed, and a filtering operation such as phase compensation and gaincompensation is performed in the microcomputer 8. The dead band areagenerating section 70 in the microcomputer 8 receives from the gainadjusting section 66 a spherical aberration correction signal obtainedafter the filtering operation. When the signal has an absolute valueexceeding the predetermined value, the microcomputer 8 outputs thesignal to a beam expander rough driving circuit 32. When the signal hasan absolute value equal to or smaller than the predetermined value, themicrocomputer 8 interrupts the output of the signal.

[0193] As will be described later, since the stepping motor is driven attime t1, the spherical aberration correction signal obtained after thefiltering operation has a waveform shown in FIG. 25(a). It is understoodthat the spherical aberration detection signal is reduced by the drivingof the stepping motor at time t1 and time t2.

[0194]FIG. 25(d) shows the output of the dead band area generatingsection 70 (spherical aberration detection signal after a dead-bandprocessing). The spherical aberration detection signal is outputted tothe beam expander rough driving circuit 32. The beam expander roughdriving circuit 32 outputs a signal of FIG. 25(b) based on the sphericalaberration correction signal obtained after the dead-band processing,and performs correction control for spherical aberration.

[0195] As shown in FIG. 25(c), the stepping motor 35 is driven by thebeam expander rough driving circuit at time t1 and t2 in such a manneras to correct spherical aberration. However, after time t2, thespherical aberration correction signal has an absolute value equal to orsmaller than the predetermined value as shown in FIG. 25(d) and theoutput is interrupted. Thus, as shown in FIG. 25(c), correction is notperformed by the stepping motor 35.

[0196] In this way, it is possible to reduce a transient error caused bythe transient response of the stepping motor 35 when the sphericalaberration correction signal (or spherical aberration detection signal)is slightly changed. Particularly when the thickness of the disc isslowly changed in a spiral operation and spherical aberration is changedat a low frequency, smooth tracking can be performed with a greateffect.

[0197] (Embodiment 6)

[0198]FIG. 26 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 6. FIG. 27 is a sectional viewshowing a light beam to explain the spherical aberration detection ofthe present embodiment. In the optical disc device of the presentembodiment, a light-receiving part 37 and a preamplifier 12 areconfigured as shown in FIG. 10 and FIG. 11 just like Embodiment 1.

[0199] In FIG. 26, reflected light received by the light-receiving part37 from an optical disc 20 is detected as photocurrent corresponding toa quantity of received light and is transmitted to the preamplifier 12.The preamplifier 12 performs current-voltage conversion and transmitsvoltage output corresponding to the photocurrent to a focus error signalgenerator 36 and a spherical aberration detector 31.

[0200] Based on a signal from the light-receiving part 37 acting aslight-receiving means, the focus error signal generator 36 acting asconverging state detecting means detects a signal corresponding to theconverging state of a light beam on an information layer 29 of theoptical disc 20. To be specific, a signal corresponding to a convergingstate is detected based on an output signal of the preamplifier 12, andan error signal of the optical disc 20 and a light beam spot, which isoutputted from an optical head 5 and is focused, is generated withrespect to the vertical direction.

[0201] The spherical aberration correction actuator 34 drives aspherical aberration correction lens unit 15 acting as sphericalaberration changing means. To be specific, a distance is adjustedbetween a pair of lenses constituting the spherical aberrationcorrection lens unit 15, so that the spherical aberration of a lightbeam spot can be changed.

[0202] In the present embodiment and later, the spherical aberrationcorrection lens unit 15 is used as spherical aberration changing means.The spherical aberration changing means is not limited in the presentinvention. An element may be applicable in which an optical distance(optical path) is changed by a liquid crystal or the like and thusspherical aberration is corrected. This kind of spherical aberrationchanging means is driven by a circuit for applying a proper voltage to aliquid crystal.

[0203] The spherical aberration detector 31 acting as sphericalaberration detecting means detects a state of spherical aberrationoccurring on a light beam spot generated on the information layer 29 ofthe optical disc 20 based on a signal from the light-receiving part 37acting as light-receiving means, and the spherical aberration detector31 outputs a signal corresponding to the state of spherical aberration(hereinafter, referred to as a spherical aberration signal).

[0204] Incidentally a focus control system and a spherical aberrationcontrol system interfere with each other. To be specific, a detectionerror corresponding to defocus appears on a spherical aberration signal,and a distance from an objective lens to the focus is changed accordingto a correction amount of spherical aberration in an FE signal. Thus,the FE signal is multiplied by a predetermined multiple in a sphericalaberration signal correcting section 132 and is added to a sphericalaberration signal, so that the influence of defocus on the sphericalaberration signal is eliminated. Hence, it is possible to cut off aninterruption loop of a focus control system and a spherical aberrationcontrol system.

[0205] The spherical aberration signal corrected based on the FE signalis transmitted to a beam expander driving circuit 133 via a sphericalaberration control section 135. Therefore, a spherical aberrationcorrection actuator 34 is controlled according to the sphericalaberration signal having been corrected based on the FE signal. Besides,the spherical aberration control section 135 has a filter for phasecompensation, gain compensation, and so on and stabilizes the sphericalaberration control system. Further, the beam expander driving circuit133 is a driving circuit of the spherical aberration correction actuator34.

[0206] Referring to FIG. 10, a method of generating the FE signal willbe described below.

[0207] A detection lens 46 converges a light beam, which is lightreflected from the optical disc 20. A polarized beam splitter 47 dividesa light beam, which is reflected light, into two. A first lightshielding plate 48 shields the light beam inside a predetermined radiusof the light beam, which is reflected light. An outer peripherallight-receiving part 40 receives a light beam having passed through thefirst light shielding plate 48, and converts the received light beaminto photocurrent. A second light shielding plate 49 shields a lightbeam outside the predetermined radius of the light beam, which isreflected light. An inner peripheral light-receiving part 41 receivesthe light beam having passed through the second light shielding plate49, and converts the received light beam into photocurrent.

[0208] To be specific, as shown in FIG. 10, in the light-receiving part37, the polarized beam splitter 47 divides a light beam serving asreflected light having passed through a detection lens 46 from theoptical disc 20. While the first light shielding plate 48 retrieves onlyan outer peripheral light beam, the second light shielding plate 49retrieves only an inner peripheral light beam. Light quantities aredetected by an outer peripheral light-receiving part 40 and an innerperipheral light-receiving part 41, respectively.

[0209] The light-receiving part 37, the focus error signal generator 36,the spherical aberration detector 31, and the preamplifier 12 of thepresent embodiment are configured as FIG. 11.

[0210] The outer peripheral light-receiving part 40 and the innerperipheral light-receiving part 41 in FIG. 11 are each divided into fourareas A, B, C, and D. Each of the areas generates photocurrent accordingto a detected light quantity and outputs the photocurrent tocorresponding I/V converters 42 a to 42 d and I/V converters 43 a to 43d that are included in the preamplifier 12. Signals converted fromcurrent to voltage by the I/V converters 42 a to 42 d and the I/Vconverters 43 a to 43 d are each transmitted to an outer peripheralfocus error signal generator 44 and an inner peripheral focus errorsignal generator 45.

[0211] An information track longitudinal direction is the tangentialdirection of a track 28 of the optical disc 20, and an optical discradius direction is a direction perpendicular to the track 28 of theoptical disc 20. Therefore, the sum of the I/V converter 42 b and theI/V converter 42 d is subtracted from the sum of the I/V converter 42 aand the I/V converter 42 c in the outer peripheral focus error signalgenerator 44, so that an outer peripheral focus error signal serving asan FE signal is obtained by the astigmatic method, and the sum of theI/V converter 43 b and the I/V converter 43 d is subtracted from the sumof the I/V converter 43 a and the I/V converter 43 c in the innerperipheral focus error signal generator 45, so that an inner peripheralfocus error signal serving as an FE signal is obtained by the astigmaticmethod.

[0212] A focus error signal actually used for focus control in thepresent embodiment is a signal obtained by adding the outer peripheralfocus error signal and the inner peripheral focus error signal in thefocus error signal generator 36. Namely, the sum of (the I/V converter42 a+the I/V converter 42 c)−(the I/V converter 42 b+the I/V converter42 d) and (the I/V converter 43 a+the I/V converter 43 c)−(the I/Vconverter 43 b+the I/V converter 43 d) can be rewritten to ((the I/Vconverter 42 a+the I/V converter 43 a)+(the I/V converter 42 c+the I/Vconverter 43 c))−((the I/V converter 42 b+the I/V converter 43 b)+(theI/V converter 42 d+the I/V converter 43 d)).

[0213] Therefore, the method of generating the focus error signalaccording to the present embodiment is somewhat different from aconventional method of generating a focus error signal according to theastigmatic method. However, the characteristics are the same.

[0214] Hence, the FE signal serving as an output signal of the focuserror signal generator 36 is used, so that a light beam spot iscontrolled so as to have a predetermined converging state on theinformation layer 29 of the optical disc 20 as in a conventional device.

[0215] Subsequently, the following will describe a generating method(detecting method) of a spherical aberration signal.

[0216] A spherical aberration signal is a signal obtained by subtractingthe inner peripheral focus error signal from the outer peripheral focuserror signal in the spherical aberration detector 31.

[0217] Referring to FIG. 27, the spherical aberration signal will bediscussed below. FIG. 27(a) shows that a distance is proper between thesurface of the disc and the information layer and causes no sphericalaberration on the information layer. FIG. 27(b) shows that the distanceis small and causes spherical aberration on the information layer.

[0218] In a state in which the focus control is performed, a light beamemitted from the optical head 5 is refracted by a substrate 21 of theoptical disc 20 as shown in FIG. 27(a), and an outer peripheral lightbeam is focused on a focal point B and an inner peripheral light beam isfocused on a focal point C. A position A is present on a straight lineconnecting the focal point B and the focal point C on the informationlayer 29. Since spherical aberration does not occur on the informationlayer 29 of the optical disc 20, the focal point B of the outerperipheral light beam and the focal point C of the inner peripherallight beam are both coincident with the position A. Namely, anequidistant surface from the position A and the wavefront of the lightbeam are coincident with each other.

[0219] As shown in FIG. 27(b), when the thickness of a substrate 21 isreduced, the influence of spherical aberration is increased, thethickness corresponding to a distance from the surface of the disc tothe information layer. Namely, the focal point B and the focal point Care separated from each other and the two focuses are placed into adefocus state with respect to the position A of the information layer 29where a light beam should converge. However, focus control is performedso that the focus error signal (the output signal of the focus errorgenerator 36) is almost 0, the signal being obtained by adding the outerperipheral focus error signal and the inner peripheral focus errorsignal. Therefore, the position A is coincident with the informationlayer 29. At this point, the wavefront of the light beam is notcoincident with the equidistant surface from the position A. In thisdrawing, solid lines indicate inner peripheral and outer peripherallight beams when spherical aberration occurs, and broken lines showinner peripheral and outer peripheral light beams when sphericalaberration does not occur. Further, when a thickness between the surfaceof the disc and the information layer becomes larger than that of FIG.27(a), the focal point B and the focal point C are similarly separatedfrom each other, and the two focuses are placed in the defocus statewith respect to the position A of the information layer 29 where a lightbeam should converge.

[0220] As shown in FIG. 11, the spherical aberration detector 31 actingas spherical aberration detecting means detects an influence amount ofspherical aberration on the outer peripheral light beam (a defocusamount on the focal point B) and an influence amount of sphericalaberration on the inner peripheral light beam (a defocus amount on thefocal point C). Then, the spherical aberration detector 31 detects,based on the influence amounts, a signal according to an amount ofspherical aberration occurring on the converging position of the lightbeam. To be specific, a difference is calculated between the outerperipheral focus error signal and the inner peripheral focus errorsignal, which are the output signals of the outer peripheral focus errorsignal generator 44 and the inner peripheral focus error signalgenerator 45, so that a spherical aberration detection signal isgenerated according to an amount of spherical aberration occurring onthe converging position of the light beam.

[0221] In FIG. 26, the spherical aberration signal is subjected to afiltering operation such as phase compensation and gain compensation inthe spherical aberration control section 135. Thereafter, the sphericalaberration control section 135 outputs a driving signal for moving thespherical aberration correction lens unit 15 to the beam expanderdriving circuit 133, and the spherical aberration correction actuator 34having received the driving signal moves the spherical aberrationcorrection lens unit 15. Namely, correction control is performed so thatspherical aberration is almost 0, that is the focal point B and thefocal point C are coincident with each other. In other words, correctioncontrol is performed so that the focal point B and the focal point C areboth brought closer to the position A. However, the focus control systemand the spherical aberration control system interfere with each other,resulting in instability in the control systems.

[0222] Referring to the waveform charts of FIGS. 49 and 50, theinterference of the focus control system and the spherical aberrationcontrol system will be described. The following will firstly describethe influence of the focus control system on the spherical aberrationsignal. It is assumed that the spherical aberration control system isnot operated. FIG. 49(a) shows that a received light beam is divided atthe 50% radius position of the received light beam by adjusting thefirst light shielding plate 48 and the second light shielding plate 49.FIG. 49(b) shows the outer peripheral focus error signal, FIG. 49(c)shows the inner peripheral focus error signal, FIG. 49(d) shows thefocus error signal, and FIG. 49(e) shows the spherical aberrationdetection signal. Additionally, as described above, a signal obtained bysubtracting the inner peripheral focus error signal of FIG. 49(c) fromthe outer peripheral focus error signal of FIG. 49(b) is the sphericalaberration detection signal of FIG. 49(e). Vertical axes representvoltages of the signals and horizontal axes represent defocus.

[0223]FIG. 50(a) shows that a received light beam is divided at the 75%radius position of the received light beam by adjusting the first lightshielding plate 48 and the second light shielding plate 49. FIG. 50(b)shows the outer peripheral focus error signal, FIG. 50(c) shows theinner peripheral focus error signal, FIG. 50(d) shows the focus errorsignal, and FIG. 50(e) shows the spherical aberration detection signal.Vertical axes represent voltages of the signals and horizontal axesrepresent defocus.

[0224] As shown in FIG. 49(a), when the received light beam is dividedon the 50% radius position of the received light beam, since the outerperiphery is larger in light quantity than the inner periphery, theouter peripheral focus error signal of FIG. 49(b) is larger in amplitudethan the inner peripheral focus error signal of FIG. 49(c). As a result,even though spherical aberration has a constant displacement, thespherical aberration detection signal is changed by defocus. Besides,the spherical aberration signal is in the same polarity (delay of 0°from the phase of the FE signal) as the focus error signal of FIG. 49(d)due to defocus.

[0225] Meanwhile, as shown in FIG. 50(a), when the received light beamis divided on the 75% radius position of the received light beam, sincethe outer periphery is smaller in light quantity than the innerperiphery, the outer peripheral focus error signal of FIG. 50(b) issmaller in amplitude than the inner peripheral focus error signal ofFIG. 50(c). As a result, even though spherical aberration has a constantdisplacement, the spherical aberration detection signal is changed bydefocus. Besides, the spherical aberration signal is opposite inpolarity (delay of 180° from the phase of the FE signal) to the focuserror signal of FIG. 50(d) due to defocus.

[0226] The displacement of the spherical aberration signal that iscaused by the above defocus acts as disturbance in the sphericalaberration control system.

[0227] Referring to FIG. 53, the following will specifically describethat the movement of the spherical aberration correction lens unit 15acts as disturbance in the focus control system. FIG. 53 is a schematicdrawing showing the influence of the position of the sphericalaberration correction lens on a distance from an objective lens to thefocus. FIG. 53(a) shows that an optimum thickness is set between thesurface of the disc and the information layer and no sphericalaberration occurs on the information layer. Similarly FIG. 53(b) shows alarger thickness. Besides, FIG. 53(b) shows that the focus controlsystem is normally operated and spherical aberration occurring on theinformation layer is corrected by the spherical aberration correctionlens unit 15. FIG. 53(c) shows a smaller thickness. FIG. 53(c) showsthat spherical aberration occurring on the information layer iscorrected by the spherical aberration correction lens unit 15 as FIG.53(b).

[0228] As shown in FIG. 53(b), as the substrate is increased inthickness, an interval W of the spherical aberration correction lensunit 15 is reduced. Further, a distance Z from the objective lens 1 tothe focus is increased.

[0229] Further, as shown in FIG. 53(c), as the substrate is reduced inthickness, the interval W is increased and the distance Z is reduced.The distance Z is changed according to a change in the interval W of thespherical aberration correction lens unit 15. Namely, the change in thedistance Z acts as disturbance in the focus control system.

[0230] The following will describe a method of removing the influence ofthe focus control system on the spherical aberration signal. Besides,the spherical aberration signal correcting section 132 is a block forremoving the influence. Referring to FIG. 28, the operation of thespherical aberration signal correcting section 13 will be describedbelow. FIG. 28(a) shows the output of the focus actuator driving circuit9. FIG. 28(b) shows the output of the focus error signal generator 36,FIG. 28(c) shows the output of the spherical aberration signalcorrecting section 132, FIG. 28(d) shows the output of the sphericalaberration detector 31, and FIG. 28(e) shows the spherical aberrationsignal after correction.

[0231] Additionally, the drawings show that disturbance with a higherfrequency than the band of the focus control system is applied to thefocus control system. The output of the focus actuator driving circuit 9serves as a focus driving signal according to disturbance applied asFIG. 28(a). Further, a defocus amount has the waveform of FIG. 28(a).The spherical aberration signal changes its level according to a defocusamount as described above and has the waveform of FIG. 28(d). FIG. 28(d)shows disturbance applied by the focus control system to the sphericalaberration signal. The microcomputer 8 multiplies the FE signal by apredetermined number (K) in the spherical aberration signal correctingsection 132 during focus control and adds the result to sphericalaberration signal, so that the influence of defocus upon the sphericalaberration signal is removed as shown in FIG. 28(e).

[0232] The following will describe a method of determining anamplification factor K of the spherical aberration signal correctingsection 132. FIG. 29 is a block diagram showing the configuration of anoptical disc device to explain the method of learning an amplificationfactor in the spherical aberration signal correcting section accordingto the present embodiment. The optical disc device of FIG. 29 is formedby adding a block for learning an amplification factor K to the opticaldisc device of FIG. 1. Therefore, a block denoted by the same referencenumeral as FIG. 29 is the same block as that of FIG. 1. The focus testsignal generator 50 adds a test signal to a focus driving signaloutputted from a focus control section 17. A first amplitude detectingsection 51 detects the amplitude of the spherical aberration signal. Aspherical aberration correction learning section 52 searches for anamplification factor of the spherical aberration signal correctingsection 132 that causes the first amplitude detecting section 51 to havea minimum amplitude detection signal.

[0233] Referring to the waveform of FIG. 30, the operation will bediscussed below. FIG. 30(a) shows the output of the focus actuatordriving circuit 9. Similarly FIG. 30(b) shows the output of the focuserror signal generator 36, FIG. 30(c) shows the amplification factor ofthe spherical aberration signal correcting section 132, FIG. 30(d) showsthe output of the spherical aberration signal correcting section 132,FIG. 30(e) shows the output of the spherical aberration detector 31,FIG. 30(f) shows a spherical aberration signal after correction, andFIG. 30(g) shows the output of the first amplitude detecting section 51.Besides, as shown in FIG. 50(a), a received light beam is divided at the75% radius of the light beam. A vertical axis represents a voltage of asignal and a horizontal axis represents time. The spherical aberrationcorrection learning section 52 sets Ka as an amplification factor of thespherical aberration signal correcting section 132, that is, acoefficient K at initial time t0.

[0234] The focus test signal generator 50 adds the test signal of FIG.30(a) to the focus driving signal, which is the output of the focuscontrol section 17, when focus control is performed and sphericalaberration control is not performed. Since the focus error signalgenerator 36 is opposite in polarity to the focus driving signal, theoutput of the focus error signal generator 36 has the signal of FIG.30(b) that has a phase shift of 180° from the phase of FIG. 30(a). Inthis state, since the amplitude of the spherical aberration signal isproportionate to the focus error signal, the spherical aberration signalhas the waveform of FIG. 30(e). However, as shown in FIG. 50(a), thespherical aberration signal is opposite in polarity to the FE signal.

[0235] While the spherical aberration correction learning section 52gradually changes the coefficient K of the spherical aberration signalcorrecting section 132 via the microcomputer 8, the spherical aberrationcorrection learning section 52 measures the amplitude of the sphericalaberration signal obtained after correction. Time t1 has a coefficientof Kb and time t2 has a coefficient of Kc. Additionally, the amplitudeof the spherical aberration signal after correction is measured by thefirst amplitude detecting section 51. In FIG. 30, when the coefficient Kis Ka and Kc, the signal of the spherical aberration signal aftercorrection does not have a minimum signal, but when the coefficient K isKb, the amplitude is minimum. Therefore, as shown in FIG. 30(g), thespherical aberration signal after correction has the minimum amplitudeat an amplification factor Kb, which is determined as an amplificationfactor of the spherical aberration signal correcting section 132.

[0236] Referring to the flowchart of FIG. 31, an operation fordetermining the amplification factor K of the spherical aberrationsignal correcting section 132 will be described below. First, thespherical aberration correction leaning section 52 sets the initialvalue Ka as an amplification factor of the spherical aberration signalcorrecting section 132 via the microcomputer 8 in step S1.

[0237] In step S2, the focus test signal generator 50 starts adding atest signal to the focus driving signal of the focus control section 17when the focus control is performed and spherical aberration control isnot performed. In step S3, the amplitude of the spherical aberrationsignal having been corrected by the spherical aberration signalcorrecting section 132 is acquired from the first amplitude detectingsection 51 and is stored as the amplitude minimum value. In step S4, apredetermined value is subtracted from the amplification factor of thespherical aberration signal correcting section 132.

[0238] In step S5, a comparison is performed to decide whether or notthe amplitude of the corrected spherical aberration signal detected bythe first amplitude detecting section 51 is smaller than the amplitudeminimum value. When the corrected spherical aberration signal hassmaller amplitude than the stored amplitude minimum value, the amplitudeof the corrected spherical aberration signal is newly stored as theamplitude minimum value in step S6, and the operation proceeds to stepS7. When the amplitude of the corrected spherical aberration signal isnot smaller than the stored amplitude minimum value, the operationproceeds to step S7. In step S7, a comparison is performed to decidewhether or not the amplification factor of the spherical aberrationsignal correcting section 132 is larger than Kc. When the amplificationfactor is larger than Kc, the operation returns to step S4. When theamplification is not larger, the operation proceeds to step S8. In stepS8, an amplification factor of the spherical aberration signalcorrecting section 132 is set so as to correspond to the storedamplitude minimum value, and thus the operation is completed.

[0239] Subsequently, the following will describe that the amplificationfactor K of the spherical aberration signal correcting section 132 isswitched for each layer when information is recorded or reproduced onthe optical disc 20 having a plurality of information layers in alaminated structure.

[0240] The following will discuss recording/reproduction on the opticaldisc 20 shown in FIG. 5. In a double-layer disc, defocus described withreference to FIGS. 49 and 50 affects a spherical aberration signaldifferently for each of the different information layers. This pointwill be discussed below in accordance with the waveform charts of FIGS.51 and 52.

[0241]FIG. 51(a) shows a division made by the first light shieldingplate 48 and the second light shielding plate 49 when recording orreproduction is performed on an information layer L0. FIG. 51(b) showsan outer peripheral focus error signal, FIG. 51(c) shows an innerperipheral focus error signal, FIG. 51(d) shows a focus error signal,and FIG. 51(e) shows a spherical aberration detection signal. Verticalaxes represent voltages of the signals and horizontal axes representdefocus.

[0242]FIG. 52(a) shows a division made by the first light shieldingplate 48 and the second light shielding plate 49 when a focus isobtained on an information layer L1. FIG. 51(b) shows the outerperipheral focus error signal, FIG. 51(c) shows the inner peripheralfocus error signal, FIG. 51(d) shows the focus error signal, and FIG.51(e) shows the spherical aberration detection signal. Vertical axesrepresent voltages of the signals and horizontal axes represent defocus.

[0243] As shown in FIG. 51(a), it is assumed that a light beam receivedwith the focus on the information layer L0 is divided at the 50% radiusof the received light beam. Therefore, FIGS. 51(b), 51(c), 51(d), and51(e) have the same waveforms as FIG. 49.

[0244] On the other hand, as shown in FIG. 52(b) with the focus on theinformation layer L1, the interval W of the spherical aberrationcorrection lens unit 15 is smaller as compared with the focus on theinformation layer L0, so that the light beam incident on the objectivelens 1 becomes diverging light. Therefore, return light has a smallerradius. The return light is reflected from the information layer, passesthrough the spherical aberration correction lens unit 15, and isincident on the light-receiving part. For example, although the firstlight shielding plate 48 and the second light shielding plate 49 areequal in adjustment amount, since the light beam is reduced in radius,an actual dividing position is larger than the 50% radius. In FIG.52(a), the division is made at the 75% radius. Thus, the outer peripheryis smaller in light quantity than the inner periphery, so that the outerperipheral focus error signal of FIG. 52(b) is smaller in amplitude thanthe inner peripheral focus error signal of FIG. 53(c).

[0245] As a result, the spherical aberration detection signal of FIG.53(e) that is a difference signal of the outer peripheral focus errorsignal and the inner peripheral focus error signal is opposite inpolarity due to defocus (delay of 180° from the phase of the FE signal)to the focus error signal of FIG. 53(d) that is an added signal of theouter peripheral focus error signal and the inner peripheral focus errorsignal. As described above, for each of the different information layerson which recording or reproduction is performed, the sphericalaberration detector 31 affects the spherical aberration signaldifferently according to the movement of the objective lens 1. Hence, itis necessary to switch the amplification factor of the sphericalaberration signal correcting section 132 for removing the influence.

[0246] Referring to FIG. 32, the following will discuss the switching ofthe amplification factor of the spherical aberration correcting sectionduring the interlayer movement of the laminated disc. FIG. 32(a) showsthe movement of a light beam spot during interlayer movement. FIG. 32(b)shows an amplification factor of the spherical aberration signalcorrecting section. A vertical axis represents voltages of signals and ahorizontal axis represents time. FIG. 32(c) represents ON/OFF ofspherical aberration control. FIG. 32(d) shows ON/OFF of focus control.A vertical axis represents ON/OFF of control, reference character Hrepresents ON, reference numeral L represents OFF, and a horizontal axisrepresents time. FIG. 32(e) shows an FE signal, and FIG. 32(f) shows afocus driving signal. A vertical axis represents voltages of the signalsand a horizontal axis represents time.

[0247] There are provided: an added gain storing section for storing anamplification factor of the spherical aberration signal correctingsection 132 for each layer, an added gain switching section whichretrieves a desired amplification factor of the spherical aberrationsignal correcting section 132 from the added gain storing section andnewly sets the amplification factor, and the microcomputer 8. It isassumed that a light beam firstly scans a given track on the L0. Thefollowing will describe an operation of reproducing data of the L1.First, the microcomputer 8 stores the amplification factor of thespherical aberration signal correcting section 132 for the L0 in theadded gain storing section and makes focus control and sphericalaberration control inoperative, that is, stops the control (time a).

[0248] Then, a predetermined acceleration/deceleration driving pulsecommand is given to the focus actuator driving circuit 9. After movementto the L1, the spherical aberration control is turned on immediatelyafter the focus control having been made inoperative is turned on (timeb). However, the spherical aberration control is not stabilized unlessthe focus control is stable. When the FE signal converges within apredetermined range while the FE signal is observed, it is decided thatthe focus control is stabilized and switching is made to theamplification factor of the spherical aberration signal correctingsection 132 for the L1 by the added gain switching section (time c).Thereafter, the spherical aberration control having been madeinoperative may be turned on (time d). Hence, it is possible toaccurately and quickly remove the influence of the spherical aberrationdetector 31 upon the spherical aberration signal relative to a travelamount of the objective lens 1 that is different in each layer withoutthe necessity for relearning in each interlayer movement, achieving agreat effect.

[0249] Moreover, after the influence of the focus control system uponthe spherical aberration signal is removed, the gain compensation of thefocus control section 17 or the spherical aberration control section 135is adjusted during focus control and spherical aberration control, sothat a gain characteristic displaced by the interference of the focuscontrol and the spherical aberration control can be also adjusted,achieving an adjustment with higher accuracy. Additionally, gaincompensation is adjusted by, for example, adding a test signal to thecontrol system and using quadrature homodyne detection.

[0250] (Embodiment 7)

[0251]FIG. 33 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 7. FIG. 34 is a waveform chart forexplaining the correction of an FE signal according to Embodiment 7. Inthese drawings, the same members and components as those of theconventional art and Embodiment 6 are indicated by the same referencenumerals and the explanation thereof is omitted.

[0252] Also in the present embodiment, focus control is performed basedon a focus error signal, which is an added signal of an outer peripheralfocus error signal and an inner peripheral focus error signal, as inEmbodiment 6. A spherical aberration signal is generated from adifference signal of the outer peripheral focus error signal and theinner peripheral focus error signal.

[0253] An FE signal correcting section 30 processes an output signal ofa beam expander driving circuit 133 and adds the output signal to an FEsignal. The FE signal correcting section 30 has a filter having the samecharacteristic as a spherical aberration correction actuator 34(hereinafter, referred to as an “equivalent filter”) and a band-passfilter. The two filters are connected in series, multiply the outputs ofthe filters by a predetermined value, and output the results. The passband of the band-pass filter is set within a frequency range higher thanthe band of a focus control system and lower than the band of aspherical aberration control system. Besides, in Embodiment 7, the bandof the focus control system is set lower than that of the sphericalaberration control system. Therefore, a frequency component which isincluded in a change in the interval of the spherical aberrationcorrection lens, is higher than the band of the focus control system,and is lower than the band of the spherical aberration control system ismultiplied by the predetermined multiple, and the result is added to theFE signal. The influence of the spherical aberration control system uponthe focus control system will be described below.

[0254] Disturbance is applied to the focus control system according to aspherical aberration correction amount in the spherical aberrationcontrol system, that is, an interval of the spherical aberrationcorrection lens. The disturbance is a change in distance from anobjective lens to a focus.

[0255] Referring to FIG. 53, the following will specifically explainthat a distance from the objective lens to the focus is changedaccording to a change in the interval of the spherical aberrationcorrection lens unit 15. FIG. 53 is a schematic diagram showing theinfluence of the interval of the spherical aberration correction lensupon a distance from the objective lens to the focus.

[0256]FIG. 53(a) shows that an optimum thickness is set between asurface of the disc to an information layer and no spherical aberrationoccurs on the information layer. Similarly FIG. 53(b) shows a largethickness. Additionally, FIG. 53(b) shows that the focus control systemis normally operated and spherical aberration occurring on theinformation layer is corrected by the spherical aberration correctionlens unit 15. FIG. 53(c) shows a small thickness. FIG. 53(c) shows thatspherical aberration occurring on the information layer is corrected bythe spherical aberration correction lens unit 15 as in FIG. 53(b).

[0257] As shown in FIG. 53(b), as a substrate is increased in thickness,an interval W of the spherical aberration correction lens unit 15 isreduced. Further, a distance Z from an objective lens 1 to the focus isincreased. Further, as shown in FIG. 53(c), as the substrate is reducedin thickness, the interval W is increased and the distance Z is reduced.The distance Z is changed according to a change in the interval W of thespherical aberration correction lens unit 15. Namely, the change in thedistance Z acts as disturbance to the focus control system.

[0258] In this way, a distance from the objective lens to the focus ischanged according to an interval of the spherical aberration correctionlens, and thus the disturbance has the same characteristic as thesurface wobbling of an optical disc 20. The focus control system has tokeep track of the disturbance. However, a frequency component of thedisturbance that is higher than the band of the focus control system isjust applied to a focus actuator 2 and increases the temperature of thefocus actuator 2, so that tracking cannot be performed.

[0259] Hence, in the FE signal correcting section 30, a frequencycomponent which is included in a change in the interval of the sphericalaberration correction lens, is higher than the band of the focus controlsystem, and is lower than the band of the spherical aberration controlsystem is multiplied by a coefficient L and is added to an FE signal, sothat the influence of a spherical aberration correction amount upon theFE signal is removed. Thus, the influence of an uneven thickness of thesubstrate of the optical disc can be removed from the focus controlsystem, the uneven thickness affecting more than the band of the focuscontrol system, and the heat of the focus actuator can be reduced.

[0260] An effective value detecting section 54 and an FE correctionlearning section 55 are blocks for determining the above coefficient L.The effective value detecting section 54 detects, from frequencycomponents included in a corrected FE signal, an effective value of acomponent higher than the band of the focus control system and lowerthan the band of the spherical aberration control system, and outputsit. The FE correction learning section 55 leans a coefficient L at whichthe effective value detecting section 54 has the minimum output.Further, the microcomputer 8 sets the value of the coefficient L for theFE signal correcting section 30.

[0261] Referring to FIG. 35, the FE signal correcting section 30 will bedescribed in detail. FIG. 35 is a block diagram showing the FE signalcorrecting section 30. An input terminal 900 is connected to the outputof the beam expander driving circuit 133. On a second input terminal904, an output signal of the FE correction learning section 55 isconnected via the microcomputer 8. A signal outputted from an outputterminal 905 is added to an FE signal which is the output of the focuserror signal generator 36.

[0262] A signal inputted to the input terminal 900 is transmitted to anequivalent filter 901. As described above, the equivalent filter 901 isa filter having the same characteristic as the spherical aberrationcorrection actuator 34. The output of the equivalent filter 901 istransmitted to a band-pass filter 902. In the following explanation, theband-pass filter will be referred to as a BPF. As described above, thepass band of the BPF 902 is a frequency range which is higher than theband of the focus control system and is lower than the band of thespherical aberration control system. The output of the BPF 902 istransmitted to a multiplier 903. The multiplier 903 multiplies thesignals of a terminal a and a terminal b and outputs the signals from aterminal c. The terminal c is sent to the output terminal 905. Theterminal b is connected to the second input terminal 904.

[0263] Since the output of the beam-expander driving circuit 133 isconnected to the input terminal 900, the output of the equivalent filter901 indicates an interval of the spherical aberration correction lens.The BPF 902 extracts a frequency component which is included in a changein the interval of the spherical aberration correction lens, is higherthan the band of the focus control system, and is lower than the band ofthe spherical aberration control system. An extracted signal and apredetermined value L, which is set by the FE correction learningsection 55, are multiplied by the multiplier 903 and are outputted fromthe output terminal 905.

[0264] This operation will be described in accordance with FIG. 34.Additionally, it is assumed that an uneven thickness of the substrate ischanged at a frequency higher than the band of the focus control systemand a frequency lower than the band of the spherical aberration controlsystem. FIG. 34(a) shows the uneven thickness of the substrate. FIG.34(b) shows the output of the beam expander driving circuit 133, FIG.34(c) shows the output of the equivalent filter 901, FIG. 34(d) showsthe output of the BPF 902, and FIG. 34(e) shows the output of the FEsignal correcting section 30, FIG. 34(f) shows the output of the focuserror signal generator 36, and FIG. 34(g) shows an FE signal aftercorrection. The vertical axis of FIG. 34(b) represents current, thevertical axes of the other waveforms represent voltage, and horizontalaxes represent time.

[0265] Beam expander driving current has the waveform of FIG. 34(b) tofollow a change in the thickness of the substrate of FIG. 34(a).Besides, the relationship between the driving current of the sphericalaberration correction actuator 34 and the interval of the correctionlens has a characteristic of a secondary oscillatory element. Therefore,at a higher frequency than each natural frequency, the interval of thecorrection lens relative to driving current has a phase lag of 180°. Forthis reason, the waveform of FIG. 34(a) and the waveform of FIG. 34(b)have a phase difference of 180°. When the driving signal of the beamexpander in FIG. 34(b) is inputted to the equivalent filter 901 of FIG.35, the output has the waveform of FIG. 34(c). Since a change in thethickness of the substrate has a frequency component lower than the bandof the spherical aberration control system, the waveform of FIG. 34(a)and the waveform of FIG. 34(c) are coincident in phase with each otherfor the above reason.

[0266] Since a change in the thickness of the substrate has a frequencycomponent within the pass band of the BPF 902, the output of the BPF 902has the same waveform of FIG. 34(d) as that of the output of theequivalent filter 901. The output of the FE signal correcting section 30has the waveform of FIG. 34(e) that is obtained by multiplying theoutput of the BPF 902 by a predetermined value.

[0267] Since a change in the thickness of the substrate has a frequencycomponent higher than the band of the focus control system, the focuscontrol system cannot keep track of the disturbance caused by a changein the interval of the spherical aberration correction lens. Therefore,the FE signal has the waveform of FIG. 34(f). The predetermined value Lset for the second input terminal 904 is adjusted by the FE correctionlearning section 55, so that the output signal amplitude of the FEsignal correcting section 30 is adjusted and the corrected FE signal hasthe waveform of FIG. 34(g) where an AC component is removed. Therefore,driving current caused by a change in the thickness of the substrate ofFIG. 34 is not applied to the focus actuator 9.

[0268] Additionally, when spherical aberration control is not performed,the spherical aberration correction lens unit 15 is stopped and theinfluence on the FE signal is eliminated. Therefore, addition is stoppedto the FE signal of the spherical aberration signal multiplied by apredetermined multiple by the FE signal correcting section 30. Thus,stable focus control can be performed.

[0269] The following will describe a method of determining thecoefficient L. A predetermined uneven thickness is required to calculatethe coefficient L. Namely, it is necessary to set an uneven thicknesschanging at a frequency higher than the band of the focus control systemand a frequency lower than the band of the spherical aberration controlsystem. However, in an actual disc, such an uneven thickness cannot beexpected all the time. Thus, the interval of the spherical aberrationcorrection lens is changed at a frequency higher than the band of thefocus control system and a frequency lower than the band of thespherical aberration control system, so that a state equivalent to thepresence of the predetermined uneven thickness can be obtained.

[0270] Referring to FIG. 54, the following will describe a state inwhich the interval of the spherical aberration correction lens ischanged at a frequency higher than the band of the focus control systemand a frequency lower than the band of the spherical aberration controlsystem. FIG. 54 is a schematic diagram showing the influence of theposition of the spherical aberration correction lens upon a distancefrom the objective lens to the focus. FIG. 54 is identical to foregoingFIG. 53, except that the substrate of the optical disc has an eventhickness in FIGS. 54(a) to 54(c).

[0271]FIG. 54(a) shows that an optimum thickness is set between thesurface of the disc and the information layer and no sphericalaberration occurs on the information layer. Similarly FIG. 54(b) showsan optimum state when the substrate originally has a large thickness.Further, FIG. 54(b) shows that the spherical aberration correction lensunit 15 is operated with a frequency component higher than the band offocus control and the focus control system does not normally performtracking and spherical aberration occurring on the information layer isnot corrected. FIG. 54(c) shows an optimum state when the substrateoriginally has a small thickness. As with FIG. 54(b), FIG. 54(c) showsthat focus control on the information layer and spherical aberrationoccurring on the information layer are not corrected. As with FIG. 53,when the interval W of the spherical aberration correction lens unit 15is reduced, the distance Z from the objective lens 1 to the focus isincreased as shown in FIG. 54(b). Moreover, as shown in FIG. 54(c), asthe interval W is increased, the distance Z is reduced.

[0272] The distance Z is changed by changing the interval W of thespherical aberration correction lens unit 15. Namely, a change in thedistance Z acts as disturbance to the focus control system. A ratio of achange in Z to a change in the interval W of the spherical aberrationcorrection lens unit 15 is almost equal to a radio of a change in Z to achange in the interval W of the spherical aberration correction lensunit 15 that is described in accordance with FIG. 53.

[0273] Additionally, the state in which the spherical aberrationcorrection lens unit 15 is operated at a frequency higher than the bandof focus control, the focus control system cannot normally performtracking, and spherical aberration occurring on the information layer isnot corrected is realized by changing an interval of the sphericalaberration correction lens at a frequency higher than the band of thefocus control system and a frequency lower than the band of thespherical aberration control system while the operation of the sphericalaberration control system is stopped.

[0274] Therefore, it is possible to achieve a state equal to a statewith the presence of a predetermined uneven thickness by changing aninterval of the spherical aberration correction lens at a frequencyhigher than the band of the focus control system and a frequency lowerthan the band of the spherical aberration control system. This operationwill be described in accordance with the waveform of FIG. 36. FIG. 36(a)shows the output of the beam expander driving circuit 133. SimilarlyFIG. 36(b) shows the output of the BPF 902 of the FE signal correctingsection 30, FIG. 36(c) shows the coefficient L outputted by the FEcorrection learning section 55 to the FE signal correcting section 30,FIG. 36(d) shows the output of the FE signal correcting section 30, FIG.36(e) shows the FE signal which is the output of the focus error signalgenerator 36, FIG. 36(f) shows the FE signal after correction, and FIG.36(g) shows the output of the effective value detecting section 54. Thevertical axis of FIG. 36(b) shows current. The vertical axes of theother waveforms represent voltages of the signals and the horizontalaxes thereof represent time.

[0275] Besides, it is assumed that spherical aberration control isstopped during the learning of the predetermined value L, the drivingsignal of the beam expander is outputted according to the output signalof the spherical aberration test signal generator 53, and the frequencyband of the signal has the same waveform as FIG. 34(a). Namely, theoutput signal of the test signal generator 53 is changed at a frequencyhigher than the band of the focus control system and a frequency lowerthan the band of the spherical aberration control system. The FEcorrection learning section 55 sets La as the coefficient L of the FEsignal correcting section 30 at initial value time t0.

[0276] At this point, spherical aberration control is stopped and thebeam expander driving circuit 133 operates according to the outputsignal of the spherical aberration test signal generator 53, so that thebeam expander driving current has the waveform of FIG. 34(a). Therefore,the output of the BPF 902 of the FE signal correcting section 30 has thewaveform of FIG. 34(b). The output of the FE signal correcting section30 has a waveform obtained by multiplying the waveform of FIG. 34(b) bythe coefficient La. Since the FE signal of FIG. 34(e) has a phase shiftof 180° from the output of the FE signal correcting section 30 shown inFIG. 34(d), the corrected FE signal is a signal of large amplitude asshown in FIG. 34(f). In this state, the output of the effective valuedetecting section 54 is Ea shown in FIG. 34(g).

[0277] The FE correction learning section 55 measures the level of theeffective value detecting section 54 while gradually changing thecoefficient L of the FE signal correcting section 30 via themicrocomputer 8. The time t1 has a coefficient Lb and the time t2 has acoefficient Lc. In FIG. 36, the effective value detecting section 54does not have the minimum output level at the coefficients La and Lc buthas the minimum level when the coefficient L is Lb.

[0278] Therefore, as shown in FIG. 34(g), the effective value detectingsection 54 has the minimum output level at the time t1 at which thecoefficient Lb is set. Namely, the corrected FE signal has the minimumamplitude at the coefficient Lb. Thus, the FE learning correctingsection 55 sets Lb as the optimum coefficient L of the FE signalcorrecting section 30. Besides, as described in FIGS. 53 and 54, thecoefficient Lb similarly operates when the spherical aberrationcorrection lens unit 15 is actually moved according to a thickness ofthe substrate of the disc by the spherical aberration control.

[0279] Referring to the flowchart of FIG. 37, an operation fordetermining the coefficient L of the FE signal correcting section 30will be described below. First, FE correction learning section 55 setsthe initial value La as a coefficient of the FE signal correctingsection 30 via the microcomputer in step S1. In step S2, the sphericalaberration test signal generator 53 starts adding a test signal to thebeam expander driving signal of the spherical aberration control section135 when focus control is performed and spherical aberration control isnot performed. In step S3, the effective value of the FE signalcorrected by the FE signal correcting section 30 is obtained from theeffective value detecting section 54 and is stored as the minimum valueof the effective value. In step S4, a predetermined value is subtractedfrom the coefficient L of the FE signal correcting section 30.

[0280] In step S5, a comparison is performed to decide whether or notthe effective value of the corrected FE signal that is detected by theeffective value detecting section 54 is smaller than the minimum valueof the stored effective value. When the effective value of the correctedFE signal is smaller than the minimum value of the stored effectivevalue, the effective value of the corrected FE signal is newly stored asthe minimum value of the effective value in step S6 and the operationproceeds to step S7. When the effective value of the corrected FE signalis not smaller than the minimum value of the stored effective value, theoperation proceeds to step S7. In step S7, a comparison is performed todecide whether or not the coefficient L of the FE signal correctingsection 30 is larger than Lc. When the coefficient L is larger, theoperation returns to step S4. When the coefficient L is not larger, theoperation proceeds to step S8. In step S8, the coefficient L of the FEsignal correcting section 30 is set which correspond to the minimumvalue of the stored effective value, and the operation is completed.

[0281] Further, the gain compensation of the focus control section 17 orthe spherical aberration control section 135 is adjusted while the focuscontrol and the spherical aberration control are performed, so that itis possible to make an adjustment including a gain characteristic of adisplacement caused by the interference of the focus control and thespherical aberration control, achieving an adjustment with a higheraccuracy.

[0282] (Embodiment 8)

[0283]FIG. 38 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 8. FIG. 39 is a characteristicdiagram for explaining jitter on spherical aberration and focus offset.In these drawings, the same members and components as those of theconventional art and Embodiment 6 are indicated by the same referencenumerals and the explanation thereof is omitted. Moreover, as Embodiment6, focus control is performed based on a focus error signal which is anadded signal of an outer peripheral focus error signal and an innerperipheral focus error signal, and a spherical aberration signal isgenerated by a difference signal of the outer peripheral focus errorsignal and the inner peripheral focus error signal.

[0284] A highpass filter 56 retrieves an AC component equal to or higherthan the rotational frequency of a disc motor 10 that is included in aspherical aberration signal.

[0285] An optical disc 20 locally has an uneven thickness, which causeshigh-frequency spherical aberration during recording and reproduction.Thus, when the band of a spherical aberration control system is the DCof the optical disc, spherical aberration remains due to the localuneven thickness. The spherical aberration increases, for example,jitter on a reproduction signal.

[0286] According to the present invention, the influence of sphericalaberration remaining as increased jitter or the like of the reproductionsignal is reduced by changing a target position of a focus controlsystem, that is, performing defocusing on purpose. The influence ofspherical aberration having a small effective value of about 20 mλrmscan be reduced by defocus of about 0.1 μm. When the control band of thefocus control system is higher than the control band of the sphericalaberration control system, it is possible to reduce the influence ofhigh-frequency spherical aberration which cannot be followed by thespherical aberration control system.

[0287] Referring to FIG. 38, the following will firstly describespherical aberration correction. A microcomputer 8 outputs a drivingsignal of a predetermined value to a beam expander driving circuit 133.The beam expander driving circuit 133 drives a spherical aberrationcorrection lens unit 15 according to the driving signal by using thespherical aberration correction actuator 34, so that correction isperformed on a DC component of spherical aberration on a light beam spotformed on the information layer of the optical disc 20.

[0288] A highpass filter 56 extracts a high-frequency component of aspherical aberration detection signal which is the output of a sphericalaberration detector 31. The extracted signal is multiplied by M and theresult is added to an FE signal which is the output of a focus errorsignal generator 36. The extracted component has a higher frequency thanthe control band of the spherical aberration control system. In thepresent embodiment, since the control band of the spherical aberrationcontrol system is DC, the highpass filter 56 removes the DC componentand outputs the result.

[0289] In the AC band, a target position of the focus control system ischanged according to the spherical aberration detection signal, that is,defocus occurs in the focus control system.

[0290] Referring to FIG. 39, the following will describe a typicalrelationship among remaining spherical aberration, defocus, and jitter.The y axis represents defocus, the x axis represents sphericalaberration, and contour lines represent jitter in FIG. 39. The innermostcontour line represents jitter j1 and the following contour linesrepresent jitter j2, jitter j3, jitter j4, and jitter j5 from the insideto the outside. Besides, the relationship of j1<j2<j3<j4<j5 isestablished.

[0291] When defocus is 0 and spherical aberration is 0, that is, atpoint A, the best performance to read information on the optical disc 20is achieved. Namely the jitter indicating the reading capability has theminimum value j0. However, the optical disc 20 actually has an uneventhickness of a high frequency in one rotation. Thus, high-frequencyspherical aberration occurs accordingly. Occurring spherical aberrationwill be referred to as s1 and s2. Therefore, spherical aberration occursbetween point a and point β, increasing jitter. Additionally, the pointa has spherical aberration of s2 and the point β has sphericalaberration of s1. Jitter varies within a range from j0 to j2. However,when defocus is changed according to spherical aberration, jitter varieswithin a range from j0 to j1. Namely when defocus is set at f1 at thepoint α and defocus is set at f2 at the point β, jitter is set at j1.Therefore, jitter is reduced by defocusing according to sphericalaberration. Hence, a coefficient M of the above highpass filter 56 isexpressed by the equation below.

M=(f2−f1)/(s2−s1)

[0292] Referring to FIG. 40, the following will describe a method ofcorrecting the influence of remaining spherical aberration bydefocusing. FIG. 40 shows that spherical aberration of the DC componentis corrected, the spherical aberration being caused by an uneventhickness of the substrate. The waveform of FIG. 40(a) indicates theuneven thickness of a substrate. FIG. 40(b) indicates the output of thespherical aberration detector 31. FIG. 40(c) indicates the output of thehighpass filter 56. FIG. 40(d) indicates the output of the focus errorsignal generator 36. A vertical axis represents voltages of the signalsand a horizontal axis represents time.

[0293] As shown in FIG. 40(a), the uneven thickness of the substrate hasa local uneven thickness of an AC component and uneven thickness of a DCcomponent on the optical disc 20. Since the microcomputer 8 correctsspherical aberration of a DC component by controlling the sphericalaberration correction actuator 34, the spherical aberration detectionsignal has only an AC component and is provided as a signal shown inFIG. 40(b). Besides, s1 and s2 correspond to s1 and s2 of FIG. 39. Thehighpass filter 56 acquires the AC component of FIG. 40(b) from thespherical aberration detection signal and multiplies the AC component byM. Therefore, the highpass filter 56 has the output of FIG. 40(c).Additionally, f1 and f2 correspond to f1 and f2 of FIG. 39. The outputsignal of the highpass filter 56 is subtracted from the FE signal andthe focus control system is operated so as to set the subtracted signalat 0. Thus, the FE signal has the waveform of FIG. 40(d). Therefore,defocus occurs according to spherical aberration and an increase injitter is suppressed.

[0294] (Embodiment 9)

[0295]FIG. 41 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 9. In these drawings, the samemembers and components as those of the conventional art and Embodiment 6are indicated by the same reference numerals and the explanation thereofis omitted. Further, as Embodiment 6, focus control is performed basedon a focus error signal which is an added signal of an outer peripheralfocus error signal and an inner peripheral focus error signal, and aspherical aberration detection signal is generated based on a differencesignal of the outer peripheral focus error signal and the innerperipheral focus error signal.

[0296] In the present embodiment, the position of an objective lens 1 iscontrolled so that an FE signal serving as the output of a focusgenerator 36 is set at 0. Moreover, the interval of a sphericalaberration correction lens unit 15 is controlled so that a sphericalaberration detection signal serving as the output of a sphericalaberration detector 31 is set at 0. The present embodiment does not havea block shown in Embodiment 6 for correcting a spherical aberrationdetection signal based on an FE signal.

[0297] Referring to the block diagram of FIG. 43, the following willdescribe the mutual interference of focus control and sphericalaberration control. FIG. 43 is a block diagram for explaining theinfluence of control band and the influence of interference according toEmbodiment 9. In these drawings, the same members and components asthose of the conventional art and Embodiment 6 are indicated by the samereference numerals and the explanation thereof is omitted. A system ofα1 indicates the interference of a spherical aberration control systemwith a focus control system. α1 indicates a ratio of a distance betweenan objective lens and a focus to a beam expander driving value. A systemof α2 indicates the interference of the focus control system with thespherical aberration control system. α2 indicates a ratio of an error ofa spherical aberration detection signal to defocus. K1 indicates thedetectivity of the focus error signal generator 36. K2 indicates thedetectivity of the spherical aberration detector 31.

[0298] As described in Embodiment 6, focus control and sphericalaberration control interfere with each other. To be specific, when adefocus amount is f3, a detecting error corresponding to defocus isK1×α2×f3. Further, when a spherical aberration correction amount is b1,a distance from the objective lens to the focus is changed to α1×b1 andturns into disturbance to the focus control system. Embodiment 6described the configuration for removing a detecting error of aspherical aberration detection signal occurring according to defocus. Inthe present embodiment, by setting the control band of focus control atten times or larger than the band of spherical aberration control,stable focus control and spherical aberration control can be achievedeven in the event of a detecting error of the spherical aberrationdetection signal occurring according to defocus.

[0299]FIGS. 42A to 42D are characteristic diagrams for explaining thecontrol bands and the influence of interference according to Embodiment9. FIGS. 44A to 44D are characteristic diagrams for explaining thecharacteristics of a control section, a driving circuit, and an actuatoraccording to Embodiment 9. Hereinafter, the characteristics will bedescribed as examples in accordance with these drawings.

[0300] Referring to FIGS. 44A to 44D, the characteristics of the controlsection, the driving circuit, and the actuator will be firstlydiscussed. FIG. 44A shows a characteristic from a focus control section17 to a focus actuator driving circuit 9. FIG. 44B shows thecharacteristic of a focus actuator 2. FIG. 44C shows a characteristicfrom a spherical aberration control section 135 to a beam expanderdriving circuit 133. FIG. 44D shows the characteristic of a sphericalaberration correction actuator 34. The upper diagrams show gaincharacteristics. A vertical axis represents gain and a horizontal axisrepresents a frequency. The lower diagrams show phase characteristics. Avertical axis represents a phase and a horizontal axis represents afrequency.

[0301] As shown in FIG. 44A, the phase compensation of focus control isperformed in the focus control section 17. A phase of 2 KHz which is thegain crossover of the focus is increased by about 45 degrees. As shownin FIG. 44B, the focus actuator 2 has a primary resonance frequency ofabout 46 Hz, and the band equal to or higher than the primary resonancefrequency has inclination of −40 dB/dec. As shown in FIG. 44C, the phasecompensation of spherical aberration control is similarly performed inthe spherical aberration control section 135, and a phase of 300 Hzwhich is the gain crossover of spherical aberration control is increasedby about 45 degrees. As shown in FIG. 44D, the spherical aberrationcorrection actuator 34 has a primary resonance frequency of about 66 Hz,and the band equal to or higher than the primary resonance frequency hasinclination of −40 dB/dec.

[0302] Referring to FIG. 42, the mutual interference of focus controland spherical aberration control will be described. FIG. 42A shows anopen loop characteristic of the focus affected by interference of the 2KHz control band of the focus and the 300 Hz control band of sphericalaberration correction. Similarly FIG. 42B shows an open loopcharacteristic of spherical aberration control. FIG. 42C shows an openloop characteristic of a focus affected by interference of the 5 KHzcontrol band of the focus and the 300 Hz control band of sphericalaberration correction. Similarly FIG. 42D shows an open loopcharacteristic of spherical aberration control. The upper diagrams showgain characteristics. Vertical axes represent gain and horizontal axesrepresent frequencies. The lower diagrams show phase characteristics.Vertical axes represent phases and horizontal axes representfrequencies.

[0303] As shown in FIGS. 42A and 42C, the control band of the focus isincreased from 2 KHz (FIG. 42A) to 5 KHz (FIG. 42C) and is separatedfrom the 300 Hz control band of spherical aberration correction, so thata frequency band affected by interference can be sufficiently higherthan the control band of spherical aberration control. To be specific, arise in gain from a frequency of about 50 Hz to 4 KHz (FIG. 42B) isshifted to about 1.3 to 11 KHz. As shown in FIG. 42D, when the risingrange of gain is close to the control band, the gain rises to around 0dB, so that oscillation is likely to occur due to some changes in gainand the influence of disturbance. However, as shown in FIG. 42D, whenthe rising range of gain is far from the control band, a rise in gain issufficiently lower than 0 dB, stabilizing the control system. Further,also when the control band of spherical aberration control is reducedfrom 300 Hz, it is possible to eliminate the influence of interferencefrom the control band of the focus. As described above, by setting thecontrol band of focus control at ten times or larger than the band ofspherical aberration control, it is possible to reduce the interferenceof the focus control system and the spherical aberration control system,achieving stable focus control and spherical aberration control.

[0304] (Embodiment 10)

[0305]FIG. 45 is a block diagram showing the configuration of an opticaldisc device according to Embodiment 10. FIG. 46 is a waveform chart forexplaining spherical aberration correction during search according toEmbodiment 10. FIG. 47 is a flowchart showing the sequence of sphericalaberration correction in movement along the radius direction accordingto Embodiment 10. In these drawings, the same members and components asthose of the conventional art and Embodiment 6 are indicated by the samereference numerals and the explanation thereof is omitted. Further, asEmbodiment 6, focus control is performed based on a focus error signalwhich is an added signal of an outer peripheral focus error signal andan inner peripheral focus error signal, and a spherical aberrationdetection signal is generated by a difference signal of the outerperipheral focus error signal and the inner peripheral focus errorsignal.

[0306] A spherical aberration detection signal, which is an outputsignal of a spherical aberration detector 31, is inputted to a sphericalaberration control section 135, and a filtering operation such as phasecompensation and gain compensation is performed by the sphericalaberration control section 135 to generate a spherical aberrationcorrection signal for correcting spherical aberration. The sphericalaberration control section 135 outputs a driving signal for moving aspherical aberration correction lens unit 15 to a beam expander drivingcircuit 133, and a spherical aberration correction actuator 34 havingreceived the driving signal moves the spherical aberration correctionlens unit 15.

[0307] Namely, correction control is performed so that sphericalaberration is almost 0, that is, a focal point B and a focal point C ofFIG. 2 are coincident with each other as described in Embodiment 6. Inother words, correction control is performed so that the focal point Band the focal point C are both brought close to a position A.

[0308] A tracking error signal generator 18 generates, based on theoutput signal of a preamplifier 11, an error signal of a track 28 and alight beam spot, which has been outputted from an optical head 5 andfocused, with respect to the radius direction of the optical disc 20.The tracking error signal generator 18 generates a tracking error signal(hereinafter, referred to as a TE signal) based on an input signalaccording to a method of detecting a tracking error, the method beinggenerally called the push-pull method. The TE signal which is the outputsignal of the tracking error signal generator 18 is subjected to afiltering operation such as phase compensation and gain compensation ina tracking control section 19. Thereafter, the TE signal is outputted toa tracking actuator driving circuit 26.

[0309] An objective lens 1 is driven by a tracking actuator 27 based ona driving signal generated by the tracking actuator driving circuit 26,the light beam spot is driven so as to scan the tracks 28 on aninformation layer 29 of the optical disc 20, and thus tracking controlis achieved.

[0310] The optical head 5 can be moved in the radius direction of theoptical disc 20 by a transfer table 60 acting as searching means.Moreover, the transfer table 60 is driven by an output signal (drivingsignal) from a transfer table driving circuit 62. However, when focuscontrol and spherical aberration control are performed and trackingcontrol is not performed, during crossing of a light beam spot over thetracks on the information layer 29, disturbance having a frequency equalto that of the TE signal is superimposed on the FE signal, resulting inunstable focus control. The present invention is devised in view of theabove problem.

[0311] Hence, when tracking control is not performed, sphericalaberration control is stopped and the spherical aberration actuator isdisplaced from the optimum position to cause spherical aberration. Alight beam on the information layer is increased in spot size by theoccurrence of spherical aberration. Thus, since the spot size becomeslarger than the pitch of a groove, the TE signal is reduced inamplitude. Therefore, disturbance superimposed on the FE signal isreduced in amplitude.

[0312] Referring to FIG. 48, this operation will be discussed below.Additionally, an uneven thickness of a substrate is changed at afrequency higher than the band of the focus control system and afrequency lower than the band of the spherical aberration controlsystem. FIG. 48(a) shows the output of the tracking error signalgenerating section 18, FIG. 48(b) shows the output of a focus errorsignal generating section 36, and FIG. 48(c) shows the output of thebeam expander driving circuit 133. The vertical axis of FIG. 48(c)represents current, the vertical axes of the other waveforms representvoltages of the signals, and a horizontal axis represents time. Besides,in a section from time t1 to time t2, the beam expander driving circuit133 has the optimum output and no spherical aberration occurs on a beamspot on the information layer of the optical disc 20. Moreover, in asection from time t2 to time t3, the output of the beam expander drivingcircuit 133 is shifted from the optimum value by a predetermined amountand spherical aberration considerably occurs on a beam spot on theinformation layer 29 of the optical disc 20.

[0313] Since the tracks on the optical disc 20 have eccentricity, anumber of tracks are crossed when tracking control is not operated. Atracking error signal has the waveform of FIG. 48(a). Since a focuserror signal is generated by the astigmatic method the light beam spotcrosses over the truck, the signal is affected by the crossing ofgrooves and has the waveform of FIG. 48(b).

[0314] Besides, in FIG. 48(b), a solid line indicates a focus errorsignal affected by the crossing of grooves and a broken line indicates afocus error signal not being affected by the crossing of grooves.

[0315] In FIG. 48(c), a section from time t1 to t2 indicates the optimumoutput causing no spherical aberration on a beam spot on the informationlayer of the optical disc 20, and a section from time t2 to time t3indicates an output shifted from the optimum value by a predeterminedamount so that spherical aberration considerably occurs on a beam spoton the information layer 29.

[0316] Since no spherical aberration occurs on the information layer inthe section from time t1 to t2, the tracking error signal has themaximum amplitude as shown FIG. 48(a). However, since sphericalaberration considerably occurs in the section from time t2 to t3, thetracking error signal is reduced in amplitude. Similarly the crossing ofgrooves occurring on the focus error signal has the maximum influence inthe section from time t1 to t2 during which no spherical aberrationoccurs on the information layer, and the influence is reduced in thesection from time t2 to t3 during which spherical aberrationconsiderably occurs.

[0317] In this way, when tracking control is not performed, sphericalaberration control is stopped, the influence of the crossing of groovesupon the focus error signal can be reduced by shifting the correctionamount of spherical aberration from the optimum position by apredetermined amount, and thus focus control is stabilized. Moreover,since it is possible to reduce a disturbance component which is theinfluence of the crossing of grooves, current flowing to the focusactuator 2 can be reduced and the focus actuator 2 can be protected froma damage caused by overcurrent on the focus actuator 2.

[0318] Referring to FIG. 46, this operation will be discussed below.FIG. 46(a) shows the position in the radius direction of a light beamspot relative to time. Similarly FIG. 46(b) shows the output of thefocus error signal generator 31, FIG. 46(c) shows the output of the beamexpander driving circuit 133, FIG. 46(d) shows the operating state ofthe tracking control section 19, and FIG. 46(e) shows an output to thetransfer table driving circuit 62. A vertical axis represents voltagesof the signals and a horizontal axis represents time.

[0319] In the radius direction movement during searching and so on, thespherical aberration control section 135 firstly stops, at time a, theoutput to the beam expander driving circuit 133 based on the output ofthe spherical aberration detector 31 according to an instruction of themicrocomputer 8 as shown in FIG. 46(c). Similarly an output value of thebeam expander driving circuit 133 is changed so as to move the sphericalaberration correction lens unit 15 to a position shifted by apredetermined value from a position having spherical aberration ofalmost 0. Then, as shown in FIG. 46(d), the tracking control section 19suspends tracking control at time b according to an instruction of themicrocomputer 8.

[0320] Subsequently, the microcomputer 8 outputs a transfer tabledriving signal to the transfer table driving circuit 62 until time c asshown in FIG. 46(e). From time b to time c, the transfer table drivingcircuit 62 moves the transfer table 60, which is loaded with the opticalhead 5, in the radius direction of the optical disc 20 based on thetransfer table driving signal transmitted from the microcomputer 8.Thus, as shown in FIG. 46(a), a light beam spot is moved from the innerperiphery to the outer periphery of the optical disc. Subsequently, asshown in FIG. 46(d), the tracking control section 19 resumes trackingcontrol at time c according to an instruction of the microcomputer 8.Finally as shown in FIG. 46(c), the spherical aberration control section135 cancels the stopping of an output to the beam expander drivingcircuit 133 at time d according to an instruction of the microcomputer8, the stopping having been performed according to the output of thespherical aberration detector 31, and the spherical aberration controlsection 135 resumes spherical aberration control.

[0321] In this way, when tracking control is not performed, thespherical aberration correction lens unit 15 is shifted by apredetermined amount to increase spherical aberration occurring on alight beam spot, so that the influence of the crossing of grooves uponthe FE signal can be reduced.

[0322] Referring to the flowchart of FIG. 47, an operation for movementin the radius direction of a light beam spot will be further discussedbelow. First in step S1, the microcomputer 8 instructs the sphericalaberration control section 135 to stop spherical aberration control andmove the spherical aberration correction lens unit 15 to a positionshifted by a predetermined value from a current control position. Instep S2, the microcomputer 8 instructs the tracking control section 19to suspend tracking control. In step S3, the microcomputer 8 outputs atransfer table driving signal to the transfer table driving circuit 62so as to move a light beam spot to a target radius position. In step S4,the microcomputer 8 instructs the tracking control section 19 to resumetracking control. In step S5, the microcomputer 8 instructs thespherical aberration control section 135 to return the sphericalaberration correction lens unit 15 having been shifted from the controlposition by the predetermined value to the control position of step S1and resume spherical aberration control, and thus the operation iscompleted.

[0323] Hence, during a search with a movement in the radius direction,it is possible to reduce the influence of the crossing of grooves uponthe FE signal, achieving stable focus control.

[0324] The above embodiments described the optical disc devices in whichdata is written on an optical disc having one or two information storagelayers or data is read from such an optical disc. The number ofinformation storage layers may be three or more.

[0325] Additionally, regarding the optical disc devices of Embodiments 6to 10, the stepping motor 35 and the spherical aberration correctionactuator which are used for the optical disc of Embodiment 1 may be usedto drive the spherical aberration correction lens. Particularly when theinformation recording surface of the optical disc has three or morelayers, the stepping motor 35 is added with effect.

INDUSTRIAL APPLICABILITY

[0326] According to an optical disc of the present invention, even whenan objective lens for emitting a light beam on the optical disc has alager NA than a conventional NA (e.g. NA is 0.85 or larger), sphericalaberration can be properly corrected, achieving recording/reproductionof data with a high density.

1. An optical disc device, comprising: light beam emitting means foremitting a light beam, converging means for converging the light beamtoward an information storage medium, a first actuator for moving theconverging means substantially perpendicularly to an information layerof the information storage medium to change a converging position of thelight beam, spherical aberration changing means for changing sphericalaberration occurring on a converging position of the light beamconverged by the converging means, a second actuator for moving thespherical aberration changing means in a relatively precise manner, athird actuator for moving the spherical aberration changing means in arelatively rough manner, light-receiving means for receiving lightreflected from the information storage medium of the light beam,converging state detecting means for detecting a signal according to aconverging state on the information layer of the information storagemedium of the light beam based on a signal of the light-receiving means,focus control means for driving the first actuator based on a signal ofthe converging state detecting means and performing control so that thelight beam is converged on a desired position of the information layerof the information storage medium, spherical aberration detecting meansfor detecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, and spherical aberration control means fordriving the second actuator and the third actuator based on a signal ofthe spherical aberration detecting means and performing control so thatspherical aberration is almost 0, wherein the third actuator moves thespherical aberration changing means at least based on a direct currentcomponent included in the signal of the spherical aberration detectingmeans, and the second actuator moves the spherical aberration changingmeans based on an alternating current component included in the signalof the spherical aberration detecting means.
 2. The optical disc deviceof claim 1, wherein the spherical aberration control means divides acontrol band so that the third actuator is driven when a change inspherical aberration is equal to or lower than a rotational frequency ofthe information storage medium, and the second actuator is driven when achange in spherical aberration is equal to or higher than the rotationalfrequency of the information storage medium.
 3. An optical disc devicefor recording data on an information storage medium having at least twolaminated information layers and/or reproducing data from theinformation storage medium, comprising: light beam emitting means foremitting a light beam, converging means for converging the light beamtoward the information storage medium, a first actuator for moving theconverging means substantially perpendicularly to the information layerof the information storage medium to change a converging position of thelight beam, spherical aberration changing means for changing sphericalaberration occurring on a converging position of the light beamconverged by the converging means, a second actuator for moving thespherical aberration changing means in a relatively precise manner, athird actuator for moving the spherical aberration changing means in arelatively rough manner, light-receiving means for receiving lightreflected from the information storage medium of the light beam,converging state detecting means for detecting a signal according to aconverging state on the information layer of the information storagemedium of the light beam based on a signal of the light-receiving means,focus control means for driving the first actuator based on a signal ofthe converging state detecting means and performing control so that thelight beam is converged on a desired position of the information layerof the information storage medium, interlayer moving means for drivingthe first actuator so as to move the converging position of the lightbeam to another information layer, spherical aberration detecting meansfor detecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, and spherical aberration control means fordriving the second actuator and the third actuator based on a signal ofthe spherical aberration detecting means and performing control so thatspherical aberration is almost 0, wherein the third actuator moves thespherical aberration changing means at least based on a direct currentcomponent included in the signal of the spherical aberration detectingmeans, the second actuator moves the spherical aberration changing meansbased on an alternating current component included in the signal of thespherical aberration detecting means, and when the converging positionof the light beam is moved to another information layer by theinterlayer moving means, the spherical aberration changing means isdriven by the third actuator so as to minimize spherical aberrationcaused by the movement.
 4. The optical disc device of claim 3, wherein asignal based on an amount of spherical aberration occurring on anotherinformation layer is applied to the third actuator as an offset when theconverging position of the light beam is moved to another informationlayer by the interlayer moving means.
 5. The optical disc device ofclaim 3, wherein an operation of the spherical aberration control meansbased on the signal of the spherical aberration detecting means is notperformed until the converging position of the light beam is moved toanother information layer by the interlayer moving means and the signalof the converging state detecting means is converged within apredetermined range.
 6. An optical disc device, comprising: an opticalhead for storing, as one unit, light beam emitting means for emitting alight beam, converging means for converging the light beam toward aninformation storage medium, a first actuator for moving the convergingmeans substantially perpendicularly to an information layer of theinformation storage medium to change a converging position of the lightbeam, spherical aberration changing means for changing sphericalaberration occurring on a converging position of the light beamconverged by the converging means, a second actuator for moving thespherical aberration changing means, a third actuator for moving thespherical aberration changing means, and light-receiving means forreceiving light reflected from the information storage medium of thelight beam, converging state detecting means for detecting a signalaccording to a converging state on the information layer of theinformation storage medium of the light beam based on a signal of thelight-receiving means, focus control means for driving the firstactuator based on a signal of the converging state detecting means andperforming control so that the light beam is converged on a desiredposition of the information layer of the information storage medium,spherical aberration detecting means for detecting a signal, based on asignal of the light-receiving means, according to an amount of sphericalaberration occurring on the converging position of the light beam on theinformation layer of the information storage medium, sphericalaberration control means for driving the second actuator and the thirdactuator based on a signal of the spherical aberration detecting meansand performing control so that spherical aberration is almost 0, andsearching means for moving the optical head in a radius direction of theinformation storage medium, wherein the third actuator moves thespherical aberration changing means at least based on a direct currentcomponent included in the signal of the spherical aberration detectingmeans, the second actuator moves the spherical aberration changing meansbased on an alternating current component included in the signal of thespherical aberration detecting means, and when the converging positionof the light beam is moved to a different radius position of theinformation storage medium by the searching means, the third actuator isdriven so as to minimize spherical aberration caused by the movement. 7.The optical disc device of claim 6, wherein a signal based on an amountof spherical aberration occurring on a radius position of anotherinformation layer is applied to the third actuator as an offset when theconverging position of the light beam is moved to a radius position ofanother information layer by the searching means.
 8. The optical discdevice of claim 6, wherein an operation of the spherical aberrationcontrol means based on the signal of the spherical aberration detectingmeans is not performed until the converging position of the light beamis moved to a radius position of another information layer by thesearching means and the signal of the converging state detecting meansis converged within a predetermined range on the radius position ofanother information layer.
 9. An optical disc device for performingrecording and reproduction on an information storage medium having atleast two information layers in a laminated structure, characterized bycomprising: light beam emitting means for emitting a light beam,converging means for converging the light beam toward the informationstorage medium, a focus actuator for moving the converging meanssubstantially perpendicularly to the information layer of theinformation storage medium to change a converging position of the lightbeam, light-receiving means for receiving light reflected from theinformation storage medium of the light beam, converging state detectingmeans for detecting a signal according to a converging state on theinformation layer of the information storage medium of the light beambased on a signal of the light-receiving means, focus control means fordriving the focus actuator based on a signal of the converging statedetecting means and performing control so that the light beam isconverged on a desired position of the information layer of theinformation storage medium, spherical aberration detecting means fordetecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, spherical aberration changing means forchanging spherical aberration occurring on the converging position ofthe light beam converged by the converging means, the change being madeby driving with an elastic body, spherical aberration control means fordriving the spherical aberration changing means based on a signal of thespherical aberration detecting means and performing control so thatspherical aberration is almost 0, offset applying means for applying anoffset to the spherical aberration changing means, and offset switchingmeans for switching an offset amount of the offset applying meansaccording to the information layer of the information storage medium.10. The optical disc device of claim 9, characterized in that when thespherical aberration control means is not operated, a predeterminedoffset is applied to the spherical aberration changing means by theoffset applying means, and when the spherical aberration control meansis operated, an offset is determined based on an average of drivingoutput of the spherical aberration changing means for a circumference ofthe information storage medium and the offset of the offset applyingmeans is switched.
 11. An optical disc device, comprising: light beamemitting means for emitting a light beam, converging means forconverging the light beam toward an information storage medium, a focusactuator for moving the converging means substantially perpendicularlyto the information layer of the information storage medium to change aconverging position of the light beam, spherical aberration changingmeans for changing spherical aberration occurring on the convergingposition of the light beam converged by the converging means,light-receiving means for receiving light reflected from the informationstorage medium of the light beam, converging state detecting means fordetecting a signal according to a converging state on the informationlayer of the information storage medium of the light beam based on asignal of the light-receiving means, focus control means for driving thefocus actuator based on a signal of the converging state detecting meansand performing control so that the light beam is converged on a desiredposition of the information layer of the information storage medium,spherical aberration detecting means for detecting a signal, based on asignal of the light-receiving means, according to an amount of sphericalaberration occurring on the converging position of the light beam on theinformation layer of the information storage medium, sphericalaberration control means for moving the spherical aberration changingmeans based on a signal of the spherical aberration detecting means andperforming control so that spherical aberration is almost 0, and deadband area generating means for preventing a signal of the sphericalaberration control means from being transmitted to the sphericalaberration changing means when the signal of the spherical aberrationcontrol means has a value within a predetermined range.
 12. An opticaldisc device, comprising: converging means for converging a light beamtoward an information storage medium, a focus actuator for moving theconverging means substantially perpendicularly to an information layerof the information storage medium, spherical aberration changing meansfor changing spherical aberration occurring on a converging position ofthe light beam converged by the converging means, driving means foroperating the spherical aberration changing means, light-receiving meansfor receiving light reflected from the information storage medium of thelight beam, converging state detecting means for detecting a signalaccording to a converging state on the information layer of theinformation storage medium of the light beam based on a signal of thelight-receiving means, focus control means for driving the focusactuator based on a signal of the converging state detecting means andperforming control so that the light beam is converged on a desiredposition of the information layer of the information storage medium,spherical aberration detecting means for detecting a signal, based on asignal of the light-receiving means, according to an amount of sphericalaberration occurring on the converging position of the light beam on theinformation layer of the information storage medium, sphericalaberration control means for driving the driving means based on a signalof the spherical aberration detecting means and performing control sothat spherical aberration is almost 0, and spherical aberration signalcorrecting means for amplifying a signal of the converging statedetecting means by a predetermined gain and then adding the signal to adetection signal of the spherical aberration detecting means.
 13. Theoptical disc device of claim 12, further comprising: first test signalgenerating means for applying a test signal to the focus actuator, firstamplitude detecting means for detecting amplitude of the detectionsignal of the spherical aberration detecting means, and sphericalaberration correction learning means for calculating an added gain ofthe spherical aberration signal correcting means so that the firstamplitude detecting means detects minimum amplitude of the sphericalaberration detecting signal in a state in which the test signal isapplied to the focus actuator by the first test signal generating means.14. The optical disc device of claim 13, wherein the sphericalaberration correction learning means learns an added gain in a state inwhich the focus control means is operated and the spherical aberrationcontrol means is not operated.
 15. The optical disc device of claim 12,wherein the spherical aberration signal correction means comprises addedgain storing means for storing an added gain for each layer in theinformation unit having information layers in a laminated structure, andadded gain switching means for retrieving an added gain corresponding toa position of the optical beam from the added gain storing means andswitching the added gain.
 16. The optical disc device of claim 12,further comprising: first test signal generating means for applying atest signal to the focus actuator, focus control gain adjusting meansfor adjusting a gain of the focus control means, second test signalgenerating means for applying a test signal to the driving means, andspherical aberration control gain adjusting means for adjusting a gainof the spherical aberration control means, wherein when the focuscontrol means and the spherical aberration control means are operated,the focus control gain adjusting means makes an adjustment based on afirst test signal generated by the first test signal generating meansand the first test signal after focus control, and the sphericalaberration control gain adjusting means makes an adjustment based on aspherical aberration test signal generated by the second test signalgenerating means and the spherical aberration test signal afterspherical aberration control.
 17. An optical disc device, comprising:converging means for converging a light beam toward an informationstorage medium, a focus actuator for moving the converging meanssubstantially perpendicularly to an information layer of the informationstorage medium, spherical aberration changing means for changingspherical aberration occurring on a converging position of the lightbeam converged by the converging means, driving means for operating thespherical aberration changing means, light-receiving means for receivinglight reflected from the information storage medium of the light beam,converging state detecting means for detecting a signal according to aconverging state on the information layer of the information storagemedium of the light beam based on a signal of the light-receiving means,focus control means for driving the focus actuator based on a signal ofthe converging state detecting means and performing control so that thelight beam is converged on a desired position of the information layerof the information storage medium, spherical aberration detecting meansfor detecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, spherical aberration control means fordriving the driving means based on a signal of the spherical aberrationdetecting means and performing control so that spherical aberration isalmost 0, and converging state detection signal correcting means foramplifying the signal of the spherical aberration detecting means by apredetermined gain and then adding the signal to the detection signal ofthe converging state detecting means.
 18. The optical disc device ofclaim 17, further comprising focus control means which does not add thedetection signal of the spherical aberration detecting means to thedetection signal of the converging state detecting means, the detectionsignal of the spherical aberration detecting means having beenmultiplied by a predetermined multiple by the converging state detectionsignal correcting means, which drives the focus actuator only based onthe detection signal of the converging state detecting means, andperforms control so that the light beam is converged on a convergingposition of the information layer of the information storage medium whenthe spherical aberration control means is not performed.
 19. The opticaldisc device of claim 17, further comprising: second test signalgenerating means for applying a test signal to the driving means, andsecond amplification detecting means for detecting amplitude of thedetection signal of the converging state detecting means, convergingstate detection correction learning means for calculating an added gainof the converging state detection signal correcting means so that aneffective value of the converging state detection signal is minimized bythe second amplitude detecting means in a state in which the test signalis applied to the driving means by the second test signal generatingmeans.
 20. The optical disc device of claim 19, wherein the convergingstate detection correction learning means is operated by the focuscontrol means and learns an added gain in a state in which the sphericalaberration control means is not operated.
 21. The optical disc device ofclaim 17, further comprising: first test signal generating means forapplying a test signal to the focus actuator, focus control gainadjusting means for adjusting a gain of the focus control means, secondtest signal generating means for applying a test signal to the drivingmeans, and spherical aberration control gain adjusting means foradjusting a gain of the spherical aberration control means, wherein whenthe focus control means and the spherical aberration control means areoperated, the focus control gain adjusting means makes an adjustmentbased on a first test signal generated by the first test signalgenerating means and the first test signal after focus control, and thespherical aberration control gain adjusting means makes an adjustmentbased on a spherical aberration test signal generated by the second testsignal generating means and the spherical aberration test signal afterspherical aberration control.
 22. An optical disc device, comprising:converging means for converging a light beam toward an informationstorage medium, a focus actuator for moving the converging meanssubstantially perpendicularly to an information layer of the informationstorage medium, spherical aberration changing means for changingspherical aberration occurring on a converging position of the lightbeam converged by the converging means, driving means for operating thespherical aberration changing means, light-receiving means for receivinglight reflected from the information storage medium of the light beam,converging state detecting means for detecting a signal according to aconverging state on the information layer of the information storagemedium of the light beam based on a signal of the light-receiving means,focus control means for driving the focus actuator based on a signal ofthe converging state detecting means and performing control so that thelight beam is converged on a desired position of the information layerof the information storage medium, spherical aberration detecting meansfor detecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, lowpass filter means for retrieving acomponent lower than a predetermined frequency of an output signal ofthe spherical aberration detecting means, spherical aberration controlmeans for driving the driving means based on a signal of the lowpassfilter means and performing control so that spherical aberration isalmost 0, highpass filter means for retrieving a component higher thanthe predetermined frequency of the output signal of the sphericalaberration detecting means, and spherical aberration signal adding meansfor adding a signal of the highpass filter means, to the signal of theconverging state detecting means.
 23. An optical disc device,comprising: converging means for converging a light beam toward aninformation storage medium, a focus actuator for moving the convergingmeans substantially perpendicularly to an information layer of theinformation storage medium, spherical aberration changing means forchanging spherical aberration occurring on a converging position of thelight beam converged by the converging means, driving means foroperating the spherical aberration changing means, light-receiving meansfor receiving light reflected from the information storage medium of thelight beam, converging state detecting means for detecting a signalaccording to a converging state on the information layer of theinformation storage medium of the light beam based on a signal of thelight-receiving means, focus control means for driving the focusactuator based on a signal of the converging state detecting means andperforming control so that the light beam is converged on a desiredposition of the information layer of the information storage medium,spherical aberration detecting means for detecting a signal, based on asignal of the light-receiving means, according to an amount of sphericalaberration occurring on the converging position of the light beam on theinformation layer of the information storage medium, and sphericalaberration control means for driving the driving means based on adetection signal of the spherical aberration detecting means andperforming control so that spherical aberration is almost 0, wherein thefocus control means has a band ten times larger than a band of thespherical aberration control means.
 24. An optical disc device,comprising: converging means for converging a light beam toward aninformation storage medium having a spiral or a concentric track, afocus actuator for moving the converging means substantiallyperpendicularly to an information layer of the information storagemedium, spherical aberration changing means for changing sphericalaberration occurring on a converging position of the light beamconverged by the converging means, driving means for operating thespherical aberration changing means, a tracking actuator for moving theconverging means in a direction of crossing the track on the informationstorage medium, light-receiving means for receiving light reflected fromthe information storage medium of the light beam, converging statedetecting means for detecting a signal according to a converging stateon the information layer of the information storage medium of the lightbeam based on a signal of the light-receiving means, focus control meansfor driving the focus actuator based on a signal of the converging statedetecting means and performing control so that the light beam isconverged on a desired position of the information layer of theinformation storage medium, spherical aberration detecting means fordetecting a signal, based on a signal of the light-receiving means,according to an amount of spherical aberration occurring on theconverging position of the light beam on the information layer of theinformation storage medium, spherical aberration control means fordriving the driving means based on a signal of the spherical aberrationdetecting means and performing control so that spherical aberration isalmost 0, track displacement detecting means for detecting a signalcorresponding to a displacement of the light beam relative to the trackof the information storage medium based on the signal of thelight-receiving means, tracking control means for driving the trackingactuator based on a signal of the track displacement detecting means andperforming control so that the light beam scans the track, transfermeans for permitting the tracking actuator to move in a radius directionof an information unit, and transfer driving means for driving thetransfer means, wherein when the transfer means is operated in a statein which the focus control means is operated and the tracking controlmeans is not operated, the spherical aberration changing means is movedby a predetermined amount.